Production rule systems are conceptually simple, but there are many subtle points involved in actually using them. Some of these points and the theoretical foundations of the Postgres rule system can be found in []. Some other database systems define active database rules. These are usually stored procedures and triggers and are implemented in Postgres as functions and triggers. The query rewrite rule system (the "rule system" from now on) is totally different from stored procedures and triggers. It modifies queries to take rules into consideration, and then passes the modified query to the query optimizer for execution. It is very powerful, and can be used for many things such as query language procedures, views, and versions. The power of this rule system is discussed in [] as well as []. To understand how the rule system works it is necessary to know when it is invoked and what it's input and results are. The rule system is located between the query parser and the optimizer. It takes the output of the parser, one querytree, and the rewrite rules from the pg_rewrite catalog, which are querytrees too with some extra information, and creates zero or many querytrees as result. So it's input and output are always things the parser itself could have produced and thus, anything it sees is basically representable as an SQL statement. Now what is a querytree? It is an internal representation of an SQL statement where the single parts that built it are stored separately. These querytrees are visible when starting the Postgres backend with debuglevel 4 and typing queries into the interactive backend interface. The rule actions in the pg_rewrite system catalog are also stored as querytrees. They are not formatted like the debug output, but they contain exactly the same information. Reading a querytree requires some experience and it was a hard time when I started to work on the rule system. I can remember that I was standing at the coffee machine and I saw the cup in a targetlist, water and coffee powder in a rangetable and all the buttons in a qualification expression. Since SQL representations of querytrees are sufficient to understand the rule system, this document will not teach how to read them. It might help to learn it and the naming conventions are required in the later following descriptions. When reading the SQL representations of the querytrees in this document it is necessary to be able to identify the parts the statement is broken into when it is in the querytree structure. The parts of a querytree are the commandtype This is a simple value telling which command (SELECT, INSERT, UPDATE, DELETE) produced the parsetree. the rangetable The rangtable is a list of relations that are used in the query. In a SELECT statement that are the relations given after the FROM keyword. Every rangetable entry identifies a table or view and tells by which name it is called in the other parts of the query. In the querytree the rangetable entries are referenced by index rather than by name, so here it doesn't matter if there are duplicate names as it would in an SQL statement. This can happen after the rangetables of rules have been merged in. The examples in this document will not have this situation. the resultrelation This is an index into the rangetable that identifies the relation where the results of the query go. SELECT queries normally don't have a result relation. The special case of a SELECT INTO is mostly identical to a CREATE TABLE, INSERT ... SELECT sequence and is not discussed separately here. On INSERT, UPDATE and DELETE queries the resultrelation is the table (or view!) where the changes take effect. the targetlist The targetlist is a list of expressions that define the result of the query. In the case of a SELECT, the expressions are what builds the final output of the query. They are the expressions between the SELECT and the FROM keywords (* is just an abbreviation for all the attribute names of a relation). DELETE queries don't need a targetlist because they don't produce any result. In fact the optimizer will add a special entry to the empty targetlist. But this is after the rule system and will be discussed later. For the rule system the targetlist is empty. In INSERT queries the targetlist describes the new rows that should go into the resultrelation. Missing columns of the resultrelation will be added by the optimizer with a constant NULL expression. It is the expressions in the VALUES clause or the ones from the SELECT clause on INSERT ... SELECT. On UPDATE queries, it describes the new rows that should replace the old ones. Here now the optimizer will add missing columns by inserting expressions that put the values from the old rows into the new one. And it will add the special entry like for DELETE too. It is the expressions from the SET attribute = expression part of the query. Every entry in the targetlist contains an expression that can be a constant value, a variable pointing to an attribute of one of the relations in the rangetable, a parameter or an expression tree made of function calls, constants, variables, operators etc. the qualification The queries qualification is an expression much like one of those contained in the targetlist entries. The result value of this expression is a boolean that tells if the operation (INSERT, UPDATE, DELETE or SELECT) for the final result row should be executed or not. It is the WHERE clause of an SQL statement. the others The other parts of the querytree like the ORDER BY clause arent of interest here. The rule system substitutes entries there while applying rules, but that doesn't have much to do with the fundamentals of the rule system. GROUP BY is a special thing when it appears in a view definition and still needs to be documented. Views in Postgres are implemented using the rule system. In fact there is absolutely no difference between a CREATE VIEW myview AS SELECT * FROM mytab; compared against the two commands CREATE TABLE myview (same attribute list as for mytab); CREATE RULE "_RETmyview" AS ON SELECT TO myview DO INSTEAD SELECT * FROM mytab; because this is exactly what the CREATE VIEW command does internally. This has some side effects. One of them is that the information about a view in the Postgres system catalogs is exactly the same as it is for a table. So for the query parsers, there is absolutely no difference between a table and a view. They are the same thing - relations. That is the important one for now. Rules ON SELECT are applied to all queries as the last step, even if the command given is an INSERT, UPDATE or DELETE. And they have different semantics from the others in that they modify the parsetree in place instead of creating a new one. So SELECT rules are described first. Currently, there could be only one action and it must be a SELECT action that is INSTEAD. This restriction was required to make rules safe enough to open them for ordinary users and it restricts rules ON SELECT to real view rules. The example for this document are two join views that do some calculations and some more views using them in turn. One of the two first views is customized later by adding rules for INSERT, UPDATE and DELETE operations so that the final result will be a view that behaves like a real table with some magic functionality. It is not such a simple example to start from and this makes things harder to get into. But it's better to have one example that covers all the points discussed step by step rather than having many different ones that might mix up in mind. The database needed to play on the examples is named al_bundy. You'll see soon why this is the database name. And it needs the procedural language PL/pgSQL installed, because we need a little min() function returning the lower of 2 integer values. We create that as CREATE FUNCTION min(integer, integer) RETURNS integer AS 'BEGIN IF $1 < $2 THEN RETURN $1; END IF; RETURN $2; END;' LANGUAGE 'plpgsql'; The real tables we need in the first two rule system descripitons are these: CREATE TABLE shoe_data ( shoename char(10), -- primary key sh_avail integer, -- available # of pairs slcolor char(10), -- preferred shoelace color slminlen float, -- miminum shoelace length slmaxlen float, -- maximum shoelace length slunit char(8) -- length unit ); CREATE TABLE shoelace_data ( sl_name char(10), -- primary key sl_avail integer, -- available # of pairs sl_color char(10), -- shoelace color sl_len float, -- shoelace length sl_unit char(8) -- length unit ); CREATE TABLE unit ( un_name char(8), -- the primary key un_fact float -- factor to transform to cm ); I think most of us wear shoes and can realize that this is really useful data. Well there are shoes out in the world that don't require shoelaces, but this doesn't make Al's life easier and so we ignore it. The views are created as CREATE VIEW shoe AS SELECT sh.shoename, sh.sh_avail, sh.slcolor, sh.slminlen, sh.slminlen * un.un_fact AS slminlen_cm, sh.slmaxlen, sh.slmaxlen * un.un_fact AS slmaxlen_cm, sh.slunit FROM shoe_data sh, unit un WHERE sh.slunit = un.un_name; CREATE VIEW shoelace AS SELECT s.sl_name, s.sl_avail, s.sl_color, s.sl_len, s.sl_unit, s.sl_len * u.un_fact AS sl_len_cm FROM shoelace_data s, unit u WHERE s.sl_unit = u.un_name; CREATE VIEW shoe_ready AS SELECT rsh.shoename, rsh.sh_avail, rsl.sl_name, rsl.sl_avail, min(rsh.sh_avail, rsl.sl_avail) AS total_avail FROM shoe rsh, shoelace rsl WHERE rsl.sl_color = rsh.slcolor AND rsl.sl_len_cm >= rsh.slminlen_cm AND rsl.sl_len_cm <= rsh.slmaxlen_cm; The CREATE VIEW command for the shoelace view (which is the simplest one we have) will create a relation shoelace and an entry in pg_rewrite that tells that there is a rewrite rule that must be applied whenever the relation shoelace is referenced in a queries rangetable. The rule has no rule qualification (discussed in the non SELECT rules since SELECT rules currently cannot have them) and it is INSTEAD. Note that rule qualifications are not the same as query qualifications! The rules action has a qualification. The rules action is one querytree that is an exact copy of the SELECT statement in the view creation command. The two extra range table entries for NEW and OLD (named *NEW* and *CURRENT* for historical reasons in the printed querytree) you can see in the pg_rewrite entry aren't of interest for SELECT rules. Now we populate unit, shoe_data and shoelace_data and Al types the first SELECT in his life: al_bundy=> INSERT INTO unit VALUES ('cm', 1.0); al_bundy=> INSERT INTO unit VALUES ('m', 100.0); al_bundy=> INSERT INTO unit VALUES ('inch', 2.54); al_bundy=> al_bundy=> INSERT INTO shoe_data VALUES al_bundy-> ('sh1', 2, 'black', 70.0, 90.0, 'cm'); al_bundy=> INSERT INTO shoe_data VALUES al_bundy-> ('sh2', 0, 'black', 30.0, 40.0, 'inch'); al_bundy=> INSERT INTO shoe_data VALUES al_bundy-> ('sh3', 4, 'brown', 50.0, 65.0, 'cm'); al_bundy=> INSERT INTO shoe_data VALUES al_bundy-> ('sh4', 3, 'brown', 40.0, 50.0, 'inch'); al_bundy=> al_bundy=> INSERT INTO shoelace_data VALUES al_bundy-> ('sl1', 5, 'black', 80.0, 'cm'); al_bundy=> INSERT INTO shoelace_data VALUES al_bundy-> ('sl2', 6, 'black', 100.0, 'cm'); al_bundy=> INSERT INTO shoelace_data VALUES al_bundy-> ('sl3', 0, 'black', 35.0 , 'inch'); al_bundy=> INSERT INTO shoelace_data VALUES al_bundy-> ('sl4', 8, 'black', 40.0 , 'inch'); al_bundy=> INSERT INTO shoelace_data VALUES al_bundy-> ('sl5', 4, 'brown', 1.0 , 'm'); al_bundy=> INSERT INTO shoelace_data VALUES al_bundy-> ('sl6', 0, 'brown', 0.9 , 'm'); al_bundy=> INSERT INTO shoelace_data VALUES al_bundy-> ('sl7', 7, 'brown', 60 , 'cm'); al_bundy=> INSERT INTO shoelace_data VALUES al_bundy-> ('sl8', 1, 'brown', 40 , 'inch'); al_bundy=> al_bundy=> SELECT * FROM shoelace; sl_name |sl_avail|sl_color |sl_len|sl_unit |sl_len_cm ----------+--------+----------+------+--------+--------- sl1 | 5|black | 80|cm | 80 sl2 | 6|black | 100|cm | 100 sl7 | 7|brown | 60|cm | 60 sl3 | 0|black | 35|inch | 88.9 sl4 | 8|black | 40|inch | 101.6 sl8 | 1|brown | 40|inch | 101.6 sl5 | 4|brown | 1|m | 100 sl6 | 0|brown | 0.9|m | 90 (8 rows) It's the simplest SELECT Al can do on our views, so we take this to explain the basics of view rules. The 'SELECT * FROM shoelace' was interpreted by the parser and produced the parsetree SELECT shoelace.sl_name, shoelace.sl_avail, shoelace.sl_color, shoelace.sl_len, shoelace.sl_unit, shoelace.sl_len_cm FROM shoelace shoelace; and this is given to the rule system. The rule system walks through the rangetable and checks if there are rules in pg_rewrite for any relation. When processing the rangetable entry for shoelace (the only one up to now) it finds the rule '_RETshoelace' with the parsetree SELECT s.sl_name, s.sl_avail, s.sl_color, s.sl_len, s.sl_unit, float8mul(s.sl_len, u.un_fact) AS sl_len_cm FROM shoelace *OLD*, shoelace *NEW*, shoelace_data s, unit u WHERE bpchareq(s.sl_unit, u.un_name); Note that the parser changed the calculation and qualification into calls to the appropriate functions. But in fact this changes nothing. The first step in rewriting is merging the two rangetables. The resulting parsetree then reads SELECT shoelace.sl_name, shoelace.sl_avail, shoelace.sl_color, shoelace.sl_len, shoelace.sl_unit, shoelace.sl_len_cm FROM shoelace shoelace, shoelace *OLD*, shoelace *NEW*, shoelace_data s, unit u; In step 2 it adds the qualification from the rule action to the parsetree resulting in SELECT shoelace.sl_name, shoelace.sl_avail, shoelace.sl_color, shoelace.sl_len, shoelace.sl_unit, shoelace.sl_len_cm FROM shoelace shoelace, shoelace *OLD*, shoelace *NEW*, shoelace_data s, unit u WHERE bpchareq(s.sl_unit, u.un_name); And in step 3 it replaces all the variables in the parsetree, that reference the rangetable entry (the one for shoelace that is currently processed) by the corresponding targetlist expressions from the rule action. This results in the final query SELECT s.sl_name, s.sl_avail, s.sl_color, s.sl_len, s.sl_unit, float8mul(s.sl_len, u.un_fact) AS sl_len_cm FROM shoelace shoelace, shoelace *OLD*, shoelace *NEW*, shoelace_data s, unit u WHERE bpchareq(s.sl_unit, u.un_name); Turning this back into a real SQL statement a human user would type reads SELECT s.sl_name, s.sl_avail, s.sl_color, s.sl_len, s.sl_unit, s.sl_len * u.un_fact AS sl_len_cm FROM shoelace_data s, unit u WHERE s.sl_unit = u.un_name; That was the first rule applied. While this was done, the rangetable has grown. So the rule system continues checking the range table entries. The next one is number 2 (shoelace *OLD*). Relation shoelace has a rule, but this rangetable entry isn't referenced in any of the variables of the parsetree, so it is ignored. Since all the remaining rangetable entries either have no rules in pg_rewrite or aren't referenced, it reaches the end of the rangetable. Rewriting is complete and the above is the final result given into the optimizer. The optimizer ignores the extra rangetable entries that aren't referenced by variables in the parsetree and the plan produced by the planner/optimizer would be exactly the same as if Al had typed the above SELECT query instead of the view selection. Now we face Al with the problem that the Blues Brothers appear in his shop and want to buy some new shoes, and as the Blues Brothers are, they want to wear the same shoes. And they want to wear them immediately, so they need shoelaces too. Al needs to know for which shoes currently in the store he has the matching shoelaces (color and size) and where the total number of exactly matching pairs is greater or equal to two. We theach him how to do and he asks his database: al_bundy=> SELECT * FROM shoe_ready WHERE total_avail >= 2; shoename |sh_avail|sl_name |sl_avail|total_avail ----------+--------+----------+--------+----------- sh1 | 2|sl1 | 5| 2 sh3 | 4|sl7 | 7| 4 (2 rows) Al is a shoe guru and so he knows that only shoes of type sh1 would fit (shoelace sl7 is brown and shoes that need brown shoelaces aren't shoes the Blues Brothers would ever wear). The output of the parser this time is the parsetree SELECT shoe_ready.shoename, shoe_ready.sh_avail, shoe_ready.sl_name, shoe_ready.sl_avail, shoe_ready.total_avail FROM shoe_ready shoe_ready WHERE int4ge(shoe_ready.total_avail, 2); The first rule applied will be that one for the shoe_ready relation and it results in the parsetree SELECT rsh.shoename, rsh.sh_avail, rsl.sl_name, rsl.sl_avail, min(rsh.sh_avail, rsl.sl_avail) AS total_avail FROM shoe_ready shoe_ready, shoe_ready *OLD*, shoe_ready *NEW*, shoe rsh, shoelace rsl WHERE int4ge(min(rsh.sh_avail, rsl.sl_avail), 2) AND (bpchareq(rsl.sl_color, rsh.slcolor) AND float8ge(rsl.sl_len_cm, rsh.slminlen_cm) AND float8le(rsl.sl_len_cm, rsh.slmaxlen_cm) ); In reality the AND clauses in the qualification will be operator nodes of type AND with a left and right expression. But that makes it lesser readable as it already is, and there are more rules to apply. So I only put them into some parantheses to group them into logical units in the order they where added and we continue with the rule for relation shoe as it is the next rangetable entry that is referenced and has a rule. The result of applying it is SELECT sh.shoename, sh.sh_avail, rsl.sl_name, rsl.sl_avail, min(sh.sh_avail, rsl.sl_avail) AS total_avail, FROM shoe_ready shoe_ready, shoe_ready *OLD*, shoe_ready *NEW*, shoe rsh, shoelace rsl, shoe *OLD*, shoe *NEW*, shoe_data sh, unit un WHERE (int4ge(min(sh.sh_avail, rsl.sl_avail), 2) AND (bpchareq(rsl.sl_color, sh.slcolor) AND float8ge(rsl.sl_len_cm, float8mul(sh.slminlen, un.un_fact)) AND float8le(rsl.sl_len_cm, float8mul(sh.slmaxlen, un.un_fact)) ) ) AND bpchareq(sh.slunit, un.un_name); And finally we apply the already well known rule for shoelace (this time on a parsetree that is a little more complex) and get SELECT sh.shoename, sh.sh_avail, s.sl_name, s.sl_avail, min(sh.sh_avail, s.sl_avail) AS total_avail FROM shoe_ready shoe_ready, shoe_ready *OLD*, shoe_ready *NEW*, shoe rsh, shoelace rsl, shoe *OLD*, shoe *NEW*, shoe_data sh, unit un, shoelace *OLD*, shoelace *NEW*, shoelace_data s, unit u WHERE ( (int4ge(min(sh.sh_avail, s.sl_avail), 2) AND (bpchareq(s.sl_color, sh.slcolor) AND float8ge(float8mul(s.sl_len, u.un_fact), float8mul(sh.slminlen, un.un_fact)) AND float8le(float8mul(s.sl_len, u.un_fact), float8mul(sh.slmaxlen, un.un_fact)) ) ) AND bpchareq(sh.slunit, un.un_name) ) AND bpchareq(s.sl_unit, u.un_name); Again we reduce it to a real SQL statement that is equivalent to the final output of the rule system: SELECT sh.shoename, sh.sh_avail, s.sl_name, s.sl_avail, min(sh.sh_avail, s.sl_avail) AS total_avail FROM shoe_data sh, shoelace_data s, unit u, unit un WHERE min(sh.sh_avail, s.sl_avail) >= 2 AND s.sl_color = sh.slcolor AND s.sl_len * u.un_fact >= sh.slminlen * un.un_fact AND s.sl_len * u.un_fact <= sh.slmaxlen * un.un_fact AND sh.sl_unit = un.un_name AND s.sl_unit = u.un_name; Recursive processing of rules rewrote one SELECT from a view into a parsetree, that is equivalent to exactly that what Al had to type if there would be no views at all. There is currently no recursion stopping mechanism for view rules in the rule system (only for the other rules). This doesn't hurt much, because the only way to push this into an endless loop (blowing up the backend until it reaches the memory limit) is to create tables and then setup the view rules by hand with CREATE RULE in such a way, that one selects from the other that selects from the one. This could never happen if CREATE VIEW is used because on the first CREATE VIEW, the second relation does not exist and thus the first view cannot select from the second. Two details of the parsetree aren't touched in the description of view rules above. These are the commandtype and the resultrelation. In fact, view rules don't need these informations. There are only a few differences between a parsetree for a SELECT and one for any other command. Obviously they have another commandtype and this time the resultrelation points to the rangetable entry where the result should go. Anything else is absolutely the same. So having two tables t1 and t2 with attributes a and b, the parsetrees for the two statements SELECT t2.b FROM t1, t2 WHERE t1.a = t2.a; UPDATE t1 SET b = t2.b WHERE t1.a = t2.a; are nearly identical. The rangetables contain entries for the tables t1 and t2. The targetlists contain one variable that points to attribute b of the rangetable entry for table t2. The qualification expressions compare the attributes a of both ranges for equality. The consequence is, that both parsetrees result in similar execution plans. They are both joins over the two tables. For the UPDATE the missing columns from t1 are added to the targetlist by the optimizer and the final parsetree will read as UPDATE t1 SET a = t1.a, b = t2.b WHERE t1.a = t2.a; and thus the executor run over the join will produce exactly the same result set as a SELECT t1.a, t2.b FROM t1, t2 WHERE t1.a = t2.a; will do. But there is a little problem in UPDATE. The executor does not care what the results from the join it is doing are meant for. It just produces a result set of rows. The difference that one is a SELECT command and the other is an UPDATE is handled in the caller of the executor. The caller still knows (looking at the parsetree) that this is an UPDATE, and he knows that this result should go into table t1. But which of the 666 rows that are there has to be replaced by the new row? The plan executed is a join with a qualification that potentially could produce any number of rows between 0 and 666 in unknown order. To resolve this problem, another entry is added to the targetlist in UPDATE and DELETE statements. The current tuple ID (ctid). This is a system attribute with a special feature. It contains the block and position in the block for the row. Knowing the table, the ctid can be used to find one specific row in a 1.5GB sized table containing millions of rows by fetching one single data block. After adding the ctid to the targetlist, the final result set could be defined as SELECT t1.a, t2.b, t1.ctid FROM t1, t2 WHERE t1.a = t2.a; Now another detail of Postgres enters the stage. At this moment, table rows aren't overwritten and this is why ABORT TRANSACTION is fast. In an UPDATE, the new result row is inserted into the table (after stripping ctid) and in the tuple header of the row that ctid pointed to the cmax and xmax entries are set to the current command counter and current transaction ID. Thus the old row is hidden and after the transaction commited the vacuum cleaner can really move it out. Knowing that all, we can simply apply view rules in absolutely the same way to any command. There is no difference. The above demonstrates how the rule system incorporates view definitions into the original parsetree. In the second example a simple SELECT from one view created a final parsetree that is a join of 4 tables (unit is used twice with different names). The benefit of implementing views with the rule system is, that the optimizer has all the information about which tables have to be scanned plus the relationships between these tables plus the restrictive qualifications from the views plus the qualifications from the original query in one single parsetree. And this is still the situation when the original query is already a join over views. Now the optimizer has to decide which is the best path to execute the query. The more information the optimizer has, the better this decision can be. And the rule system as implemented in Postgres ensures, that this is all information available about the query up to now. There was a long time where the Postgres rule system was considered broken. The use of rules was not recommended and the only part working was view rules. And also these view rules gave problems because the rule system wasn't able to apply them properly on statements other than a SELECT (for example an UPDATE that used data from a view didn't work). During that time, development moved on and many features were added to the parser and optimizer. The rule system got more and more out of sync with their capabilities and it became harder and harder to start fixing it. Thus, no one did. For 6.4, someone locked the door, took a deep breath and shuffled that damned thing up. What came out was a rule system with the capabilities described in this document. But there are still some constructs not handled and some where it fails due to things that are currently not supported by the Postgres query optimizer. Views with aggregate columns have bad problems. Aggregate expressions in qualifications must be used in subselects. Currently it is not possible to do a join of two views, each having an aggregate column, and compare the two aggregate values in the qualification. In the meantime it is possible to put these aggregate expressions into functions with the appropriate arguments and use them in the view definition. Views of unions are currently not supported. Well it's easy to rewrite a simple SELECT into a union. But it is a little difficult if the view is part of a join doing an update. ORDER BY clauses in view definitions aren't supported. DISTINCT isn't supported in view definitions. There is no good reason why the optimizer should not handle parsetree constructs that the parser could never produce due to limitations in the SQL syntax. The author hopes that these items disappear in the future. Using the described rule system to implement views has a funny side effect. The following does not seem to work: al_bundy=> INSERT INTO shoe (shoename, sh_avail, slcolor) al_bundy-> VALUES ('sh5', 0, 'black'); INSERT 20128 1 al_bundy=> SELECT shoename, sh_avail, slcolor FROM shoe_data; shoename |sh_avail|slcolor ----------+--------+---------- sh1 | 2|black sh3 | 4|brown sh2 | 0|black sh4 | 3|brown (4 rows) The interesting thing is that the return code for INSERT gave us an object ID and told that 1 row has been inserted. But it doesn't appear in shoe_data. Looking into the database directory we can see, that the database file for the view relation shoe seems now to have a data block. And that is definitely the case. We can also issue a DELETE and if it does not have a qualification, it tells us that rows have been deleted and the next vacuum run will reset the file to zero size. The reason for that behaviour is, that the parsetree for the INSERT does not reference the shoe relation in any variable. The targetlist contains only constant values. So there is no rule to apply and it goes down unchanged into execution and the row is inserted. And so for the DELETE. To change this we can define rules that modify the behaviour of non-SELECT queries. This is the topic of the next section. Rules that are defined ON INSERT, UPDATE and DELETE are totally different from the view rules described in the previous section. First, their CREATE RULE command allows more: They can have no action. They can have multiple actions. The keyword INSTEAD is optional. The pseudo relations NEW and OLD become useful. They can have rule qualifications. Second, they don't modify the parsetree in place. Instead they create zero or many new parsetrees and can throw away the original one. Keep the syntax CREATE RULE rule_name AS ON event TO object [WHERE rule_qualification] DO [INSTEAD] [action | (actions) | NOTHING]; in mind. In the following, "update rules" means rules that are defined ON INSERT, UPDATE or DELETE. Update rules get applied by the rule system when the result relation and the commandtype of a parsetree are equal to the object and event given in the CREATE RULE command. For update rules, the rule system creates a list of parsetrees. Initially the parsetree list is empty. There can be zero (NOTHING keyword), one or multiple actions. To simplify, we look at a rule with one action. This rule can have a qualification or not and it can be INSTEAD or not. What is a rule qualification? It is a restriction that tells when the actions of the rule should be done and when not. This qualification can only reference the NEW and/or OLD pseudo relations which are basically the relation given as object (but with a special meaning). So we have four cases that produce the following parsetrees for a one-action rule. No qualification and not INSTEAD: The parsetree from the rule action where the original parsetrees qualification has been added. No qualification but INSTEAD: The parsetree from the rule action where the original parsetrees qualification has been added. Qualification given and not INSTEAD: The parsetree from the rule action where the rule qualification and the original parsetrees qualification have been added. Qualification given and INSTEAD: The parsetree from the rule action where the rule qualification and the original parsetrees qualification have been added. The original parsetree where the negated rule qualification has been added. Finally, if the rule is not INSTEAD, the unchanged original parsetree is added to the list. Since only qualified INSTEAD rules already add the original parsetree, we end up with a total maximum of two parsetrees for a rule with one action. The parsetrees generated from rule actions are thrown into the rewrite system again and maybe more rules get applied resulting in more or less parsetrees. So the parsetrees in the rule actions must have either another commandtype or another resultrelation. Otherwise this recursive process will end up in a loop. There is a compiled in recursion limit of currently 10 iterations. If after 10 iterations there are still update rules to apply the rule system assumes a loop over multiple rule definitions and aborts the transaction. The parsetrees found in the actions of the pg_rewrite system catalog are only templates. Since they can reference the rangetable entries for NEW and OLD, some substitutions have to be made before they can be used. For any reference to NEW, the targetlist of the original query is searched for a corresponding entry. If found, that entries expression is placed into the reference. Otherwise NEW means the same as OLD. Any reference to OLD is replaced by a reference to the rangetable entry which is the resultrelation. We want to trace changes to the sl_avail column in the shoelace_data relation. So we setup a log table and a rule that writes us entries every time and UPDATE is performed on shoelace_data. CREATE TABLE shoelace_log ( sl_name char(10), -- shoelace changed sl_avail integer, -- new available value log_who name, -- who did it log_when datetime -- when ); CREATE RULE log_shoelace AS ON UPDATE TO shoelace_data WHERE NEW.sl_avail != OLD.sl_avail DO INSERT INTO shoelace_log VALUES ( NEW.sl_name, NEW.sl_avail, getpgusername(), 'now'::text ); One interesting detail is the casting of 'now' in the rules INSERT action to type text. Without that, the parser would see at CREATE RULE time, that the target type in shoelace_log is a datetime and tries to make a constant from it - with success. So a constant datetime value would be stored in the rule action and all log entries would have the time of the CREATE RULE statement. Not exactly what we want. The casting causes that the parser constructs a datetime('now'::text) from it and this will be evaluated when the rule is executed. Now Al does al_bundy=> UPDATE shoelace_data SET sl_avail = 6 al_bundy-> WHERE sl_name = 'sl7'; and we look at the logtable. al_bundy=> SELECT * FROM shoelace_log; sl_name |sl_avail|log_who|log_when ----------+--------+-------+-------------------------------- sl7 | 6|Al |Tue Oct 20 16:14:45 1998 MET DST (1 row) That's what we expected. What happened in the background is the following. The parser created the parsetree (this time the parts of the original parsetree are highlighted because the base of operations is the rule action for update rules). UPDATE shoelace_data SET sl_avail = 6 FROM shoelace_data shoelace_data WHERE bpchareq(shoelace_data.sl_name, 'sl7'); There is a rule 'log_shoelace' that is ON UPDATE with the rule qualification expression int4ne(NEW.sl_avail, OLD.sl_avail) and one action INSERT INTO shoelace_log SELECT *NEW*.sl_name, *NEW*.sl_avail, getpgusername(), datetime('now'::text) FROM shoelace_data *NEW*, shoelace_data *OLD*, shoelace_log shoelace_log; Don't trust the output of the pg_rules system view. It specially handles the situation that there are only references to NEW and OLD in the INSERT and outputs the VALUES format of INSERT. In fact there is no difference between an INSERT ... VALUES and an INSERT ... SELECT on parsetree level. They both have rangetables, targetlists and maybe qualifications etc. The optimizer later decides, if to create an execution plan of type result, seqscan, indexscan, join or whatever for that parsetree. If there are no references to rangetable entries leftin the parsetree , it becomes a result execution plan (the INSERT ... VALUES version). The rule action above can truely result in both variants. The rule is a qualified non-INSTEAD rule, so the rule system has to return two parsetrees. The modified rule action and the original parsetree. In the first step the rangetable of the original query is incorporated into the rules action parsetree. This results in INSERT INTO shoelace_log SELECT *NEW*.sl_name, *NEW*.sl_avai, getpgusername(), datetime('now'::text) FROM shoelace_data shoelace_data, shoelace_data *NEW*, shoelace_data *OLD*, shoelace_log shoelace_log; In step 2 the rule qualification is added to it, so the result set is restricted to rows where sl_avail changes. INSERT INTO shoelace_log SELECT *NEW*.sl_name, *NEW*.sl_avai, getpgusername(), datetime('now'::text) FROM shoelace_data shoelace_data, shoelace_data *NEW*, shoelace_data *OLD*, shoelace_log shoelace_log WHERE int4ne(*NEW*.sl_avail, *OLD*.sl_avail); In step 3 the original parsetrees qualification is added, restricting the resultset further to only the rows touched by the original parsetree. INSERT INTO shoelace_log SELECT *NEW*.sl_name, *NEW*.sl_avai, getpgusername(), datetime('now'::text) FROM shoelace_data shoelace_data, shoelace_data *NEW*, shoelace_data *OLD*, shoelace_log shoelace_log WHERE int4ne(*NEW*.sl_avail, *OLD*.sl_avail) AND bpchareq(shoelace_data.sl_name, 'sl7'); Step 4 substitutes NEW references by the targetlist entries from the original parsetree or with the matching variable references from the result relation. INSERT INTO shoelace_log SELECT shoelace_data.sl_name, 6, getpgusername(), datetime('now'::text) FROM shoelace_data shoelace_data, shoelace_data *NEW*, shoelace_data *OLD*, shoelace_log shoelace_log WHERE int4ne(6, *OLD*.sl_avail) AND bpchareq(shoelace_data.sl_name, 'sl7'); Step 5 replaces OLD references into resultrelation references. INSERT INTO shoelace_log SELECT shoelace_data.sl_name, 6, getpgusername(), datetime('now'::text) FROM shoelace_data shoelace_data, shoelace_data *NEW*, shoelace_data *OLD*, shoelace_log shoelace_log WHERE int4ne(6, shoelace_data.sl_avail) AND bpchareq(shoelace_data.sl_name, 'sl7'); That's it. So reduced to the max the return from the rule system is a list of two parsetrees that are the same as the statements: INSERT INTO shoelace_log SELECT shoelace_data.sl_name, 6, getpgusername(), 'now' FROM shoelace_data WHERE 6 != shoelace_data.sl_avail AND shoelace_data.sl_name = 'sl7'; UPDATE shoelace_data SET sl_avail = 6 WHERE sl_name = 'sl7'; These are executed in this order and that is exactly what the rule defines. The subtitutions and the qualifications added ensure, that if the original query would be an UPDATE shoelace_data SET sl_color = 'green' WHERE sl_name = 'sl7'; No log entry would get written because due to the fact that this time the original parsetree does not contain a targetlist entry for sl_avail, NEW.sl_avail will get replaced by shoelace_data.sl_avail resulting in the extra query INSERT INTO shoelace_log SELECT shoelace_data.sl_name, shoelace_data.sl_avail, getpgusername(), 'now' FROM shoelace_data WHERE shoelace_data.sl_avail != shoelace_data.sl_avail AND shoelace_data.sl_name = 'sl7'; and that qualification will never be true. Since the is no difference on parsetree level between an INSERT ... SELECT, and an INSERT ... VALUES, it will also work if the original query modifies multiple rows. So if Al would issue the command UPDATE shoelace_data SET sl_avail = 0 WHERE sl_color = 'black'; four rows in fact get updated (sl1, sl2, sl3 and sl4). But sl3 already has sl_avail = 0. This time, the original parsetrees qualification is different and that results in the extra parsetree INSERT INTO shoelace_log SELECT shoelace_data.sl_name, 0, getpgusername(), 'now' FROM shoelace_data WHERE 0 != shoelace_data.sl_avail AND shoelace_data.sl_color = 'black'; This parsetree will surely insert three new log entries. And that's absolutely correct. It is important, that the original parsetree is executed last. The Postgres "traffic cop" does a command counter increment between the execution of the two parsetrees so the second one can see changes made by the first. If the UPDATE would have been executed first, all the rows are already set to zero, so the logging INSERT would not find any row where 0 != shoelace_data.sl_avail. A simple way to protect view relations from the mentioned possibility that someone can INSERT, UPDATE and DELETE invisible data on them is to let those parsetrees get thrown away. We create the rules CREATE RULE shoe_ins_protect AS ON INSERT TO shoe DO INSTEAD NOTHING; CREATE RULE shoe_upd_protect AS ON UPDATE TO shoe DO INSTEAD NOTHING; CREATE RULE shoe_del_protect AS ON DELETE TO shoe DO INSTEAD NOTHING; If Al now tries to do any of these operations on the view relation shoe, the rule system will apply the rules. Since the rules have no actions and are INSTEAD, the resulting list of parsetrees will be empty and the whole query will become nothing because there is nothing left to be optimized or executed after the rule system is done with it. This fact might irritate frontend applications because absolutely nothing happened on the database and thus, the backend will not return anything for the query. Not even a PGRES_EMPTY_QUERY or so will be available in libpq. In psql, nothing happens. This might change in the future. A more sophisticated way to use the rule system is to create rules that rewrite the parsetree into one that does the right operation on the real tables. To do that on the shoelace view, we create the following rules: CREATE RULE shoelace_ins AS ON INSERT TO shoelace DO INSTEAD INSERT INTO shoelace_data VALUES ( NEW.sl_name, NEW.sl_avail, NEW.sl_color, NEW.sl_len, NEW.sl_unit); CREATE RULE shoelace_upd AS ON UPDATE TO shoelace DO INSTEAD UPDATE shoelace_data SET sl_name = NEW.sl_name, sl_avail = NEW.sl_avail, sl_color = NEW.sl_color, sl_len = NEW.sl_len, sl_unit = NEW.sl_unit WHERE sl_name = OLD.sl_name; CREATE RULE shoelace_del AS ON DELETE TO shoelace DO INSTEAD DELETE FROM shoelace_data WHERE sl_name = OLD.sl_name; Now there is a pack of shoelaces arriving in Al's shop and it has a big partlist. Al is not that good in calculating and so we don't want him to manually update the shoelace view. Instead we setup two little tables, one where he can insert the items from the partlist and one with a special trick. The create commands for anything are: CREATE TABLE shoelace_arrive ( arr_name char(10), arr_quant integer ); CREATE TABLE shoelace_ok ( ok_name char(10), ok_quant integer ); CREATE RULE shoelace_ok_ins AS ON INSERT TO shoelace_ok DO INSTEAD UPDATE shoelace SET sl_avail = sl_avail + NEW.ok_quant WHERE sl_name = NEW.ok_name; Now Al can sit down and do whatever until al_bundy=> SELECT * FROM shoelace_arrive; arr_name |arr_quant ----------+--------- sl3 | 10 sl6 | 20 sl8 | 20 (3 rows) is exactly that what's on the part list. We take a quick look at the current data, al_bundy=> SELECT * FROM shoelace ORDER BY sl_name; sl_name |sl_avail|sl_color |sl_len|sl_unit |sl_len_cm ----------+--------+----------+------+--------+--------- sl1 | 5|black | 80|cm | 80 sl2 | 6|black | 100|cm | 100 sl7 | 6|brown | 60|cm | 60 sl3 | 0|black | 35|inch | 88.9 sl4 | 8|black | 40|inch | 101.6 sl8 | 1|brown | 40|inch | 101.6 sl5 | 4|brown | 1|m | 100 sl6 | 0|brown | 0.9|m | 90 (8 rows) move the arrived shoelaces in al_bundy=> INSERT INTO shoelace_ok SELECT * FROM shoelace_arrive; and check the results al_bundy=> SELECT * FROM shoelace ORDER BY sl_name; sl_name |sl_avail|sl_color |sl_len|sl_unit |sl_len_cm ----------+--------+----------+------+--------+--------- sl1 | 5|black | 80|cm | 80 sl2 | 6|black | 100|cm | 100 sl7 | 6|brown | 60|cm | 60 sl4 | 8|black | 40|inch | 101.6 sl3 | 10|black | 35|inch | 88.9 sl8 | 21|brown | 40|inch | 101.6 sl5 | 4|brown | 1|m | 100 sl6 | 20|brown | 0.9|m | 90 (8 rows) al_bundy=> SELECT * FROM shoelace_log; sl_name |sl_avail|log_who|log_when ----------+--------+-------+-------------------------------- sl7 | 6|Al |Tue Oct 20 19:14:45 1998 MET DST sl3 | 10|Al |Tue Oct 20 19:25:16 1998 MET DST sl6 | 20|Al |Tue Oct 20 19:25:16 1998 MET DST sl8 | 21|Al |Tue Oct 20 19:25:16 1998 MET DST (4 rows) It's a long way from the one INSERT ... SELECT to these results. And it's description will be the last in this document (but not the last example :-). First there was the parsers output INSERT INTO shoelace_ok SELECT shoelace_arrive.arr_name, shoelace_arrive.arr_quant FROM shoelace_arrive shoelace_arrive, shoelace_ok shoelace_ok; Now the first rule 'shoelace_ok_ins' is applied and turns it into UPDATE shoelace SET sl_avail = int4pl(shoelace.sl_avail, shoelace_arrive.arr_quant) FROM shoelace_arrive shoelace_arrive, shoelace_ok shoelace_ok, shoelace_ok *OLD*, shoelace_ok *NEW*, shoelace shoelace WHERE bpchareq(shoelace.sl_name, showlace_arrive.arr_name); and throws away the original INSERT on shoelace_ok. This rewritten query is passed to the rule system again and the second applied rule 'shoelace_upd' produced UPDATE shoelace_data SET sl_name = shoelace.sl_name, sl_avail = int4pl(shoelace.sl_avail, shoelace_arrive.arr_quant), sl_color = shoelace.sl_color, sl_len = shoelace.sl_len, sl_unit = shoelace.sl_unit FROM shoelace_arrive shoelace_arrive, shoelace_ok shoelace_ok, shoelace_ok *OLD*, shoelace_ok *NEW*, shoelace shoelace, shoelace *OLD*, shoelace *NEW*, shoelace_data showlace_data WHERE bpchareq(shoelace.sl_name, showlace_arrive.arr_name) AND bpchareq(shoelace_data.sl_name, shoelace.sl_name); Again it's an INSTEAD rule and the previous parsetree is trashed. Note that this query sill uses the view shoelace But the rule system isn't finished with this loop so it continues and applies the rule '_RETshoelace' on it and we get UPDATE shoelace_data SET sl_name = s.sl_name, sl_avail = int4pl(s.sl_avail, shoelace_arrive.arr_quant), sl_color = s.sl_color, sl_len = s.sl_len, sl_unit = s.sl_unit FROM shoelace_arrive shoelace_arrive, shoelace_ok shoelace_ok, shoelace_ok *OLD*, shoelace_ok *NEW*, shoelace shoelace, shoelace *OLD*, shoelace *NEW*, shoelace_data showlace_data, shoelace *OLD*, shoelace *NEW*, shoelace_data s, unit u WHERE bpchareq(s.sl_name, showlace_arrive.arr_name) AND bpchareq(shoelace_data.sl_name, s.sl_name); Again an update rule has been applied and so the wheel turns on and we are in rewrite round 3. This time rule 'log_shoelace' gets applied what produces the extra parsetree INSERT INTO shoelace_log SELECT s.sl_name, int4pl(s.sl_avail, shoelace_arrive.arr_quant), getpgusername(), datetime('now'::text) FROM shoelace_arrive shoelace_arrive, shoelace_ok shoelace_ok, shoelace_ok *OLD*, shoelace_ok *NEW*, shoelace shoelace, shoelace *OLD*, shoelace *NEW*, shoelace_data showlace_data, shoelace *OLD*, shoelace *NEW*, shoelace_data s, unit u, shoelace_data *OLD*, shoelace_data *NEW* shoelace_log shoelace_log WHERE bpchareq(s.sl_name, showlace_arrive.arr_name) AND bpchareq(shoelace_data.sl_name, s.sl_name); AND int4ne(int4pl(s.sl_avail, shoelace_arrive.arr_quant), s.sl_avail); After that the rule system runs out of rules and returns the generated parsetrees. So we end up with two final parsetrees that are equal to the SQL statements INSERT INTO shoelace_log SELECT s.sl_name, s.sl_avail + shoelace_arrive.arr_quant, getpgusername(), 'now' FROM shoelace_arrive shoelace_arrive, shoelace_data shoelace_data, shoelace_data s WHERE s.sl_name = shoelace_arrive.arr_name AND shoelace_data.sl_name = s.sl_name AND s.sl_avail + shoelace_arrive.arr_quant != s.sl_avail; UPDATE shoelace_data SET sl_avail = shoelace_data.sl_avail + shoelace_arrive.arr_quant FROM shoelace_arrive shoelace_arrive, shoelace_data shoelace_data, shoelace_data s WHERE s.sl_name = shoelace_arrive.sl_name AND shoelace_data.sl_name = s.sl_name; The result is that data coming from one relation inserted into another, changed into updates on a third, changed into updating a fourth plus logging that final update in a fifth gets reduced into two queries. There is a little detail that's a bit ugly. Looking at the two queries turns out, that the shoelace_data relation appears twice in the rangetable where it could definitely be reduced to one. The optimizer does not handle it and so the execution plan for the rule systems output of the INSERT will be Nested Loop -> Merge Join -> Seq Scan -> Sort -> Seq Scan on s -> Seq Scan -> Sort -> Seq Scan on shoelace_arrive -> Seq Scan on shoelace_data while omitting the extra rangetable entry would result in a Merge Join -> Seq Scan -> Sort -> Seq Scan on s -> Seq Scan -> Sort -> Seq Scan on shoelace_arrive that totally produces the same entries in the log relation. Thus, the rule system caused one extra scan on the shoelace_data relation that is absolutely not necessary. And the same obsolete scan is done once more in the UPDATE. But it was a really hard job to make that all possible at all. A final demonstration of the Postgres rule system and it's power. There is a cute blonde that sells shoelaces. And what Al could never realize, she's not only cute, she's smart too - a little too smart. Thus, it happens from time to time that Al orders shoelaces that are absolutely not sellable. This time he ordered 1000 pairs of magenta shoelaces and since another kind is currently not available but he committed to buy some, he also prepared his database for pink ones. al_bundy=> INSERT INTO shoelace VALUES al_bundy-> ('sl9', 0, 'pink', 35.0, 'inch', 0.0); al_bundy=> INSERT INTO shoelace VALUES al_bundy-> ('sl10', 1000, 'magenta', 40.0, 'inch', 0.0); Since this happens often, we must lookup for shoelace entries, that fit for absolutely no shoe sometimes. We could do that in a complicated statement every time, or we can setup a view for it. The view for this is CREATE VIEW shoelace_obsolete AS SELECT * FROM shoelace WHERE NOT EXISTS (SELECT shoename FROM shoe WHERE slcolor = sl_color); It's output is al_bundy=> SELECT * FROM shoelace_obsolete; sl_name |sl_avail|sl_color |sl_len|sl_unit |sl_len_cm ----------+--------+----------+------+--------+--------- sl9 | 0|pink | 35|inch | 88.9 sl10 | 1000|magenta | 40|inch | 101.6 For the 1000 magenta shoelaces we must debt Al before we can throw 'em away, but that's another problem. The pink entry we delete. To make it a little harder for Postgres, we don't delete it directly. Instead we create one more view CREATE VIEW shoelace_candelete AS SELECT * FROM shoelace_obsolete WHERE sl_avail = 0; and do it this way: DELETE FROM shoelace WHERE EXISTS (SELECT * FROM shoelace_candelete WHERE sl_name = shoelace.sl_name); Voila: al_bundy=> SELECT * FROM shoelace; sl_name |sl_avail|sl_color |sl_len|sl_unit |sl_len_cm ----------+--------+----------+------+--------+--------- sl1 | 5|black | 80|cm | 80 sl2 | 6|black | 100|cm | 100 sl7 | 6|brown | 60|cm | 60 sl4 | 8|black | 40|inch | 101.6 sl3 | 10|black | 35|inch | 88.9 sl8 | 21|brown | 40|inch | 101.6 sl10 | 1000|magenta | 40|inch | 101.6 sl5 | 4|brown | 1|m | 100 sl6 | 20|brown | 0.9|m | 90 (9 rows) A DELETE on a view, with a subselect qualification that in total uses 4 nesting/joined views, where one of them itself has a subselect qualification containing a view and where calculated view columns are used, gets rewritten into one single parsetree that deletes the requested data from a real table. I think there are only a few situations out in the real world, where such a construct is necessary. But it makes me feel comfortable that it works. Doing this I found one more bug while writing this document. But after fixing that I was a little amazed that it works at all. Due to rewriting of queries by the Postgres rule system, other tables/views than those used in the original query get accessed. Using update rules, this can include write access to tables. Rewrite rules don't have a separate owner. The owner of a relation (table or view) is automatically the owner of the rewrite rules that are defined for it. The Postgres rule system changes the behaviour of the default access control system. Relations that are used due to rules get checked during the rewrite against the permissions of the relation owner, the rule is defined on. This means, that a user does only need the required permissions for the tables/views he names in his queries. For example: A user has a list of phone numbers where some of them are private, the others are of interest for the secretary of the office. He can construct the following: CREATE TABLE phone_data (person text, phone text, private bool); CREATE VIEW phone_number AS SELECT person, phone FROM phone_data WHERE NOT private; GRANT SELECT ON phone_number TO secretary; Nobody except him (and the database superusers) can access the phone_data table. But due to the GRANT, the secretary can SELECT from the phone_number view. The rule system will rewrite the SELECT from phone_number into a SELECT from phone_data and add the qualification that only entries where private is false are wanted. Since the user is the owner of phone_number, the read access to phone_data is now checked against his permissions and the query is considered granted. The check for accessing phone_number is still performed, so nobody than the secretary can use it. The permissions are checked rule by rule. So the secretary is for now the only one who can see the public phone numbers. But the secretary can setup another view and grant access to that to public. Then, anyone can see the phone_number data through the secretaries view. What the secretary cannot do is to create a view that directly accesses phone_data (actually he can, but it will not work since every access aborts the transaction during the permission checks). And as soon as the user will notice, that the secretary opened his phone_number view, he can REVOKE his access. Immediately any access to the secretaries view will fail. Someone might think that this rule by rule checking is a security hole, but in fact it isn't. If this would not work, the secretary could setup a table with the same columns as phone_number and copy the data to there once per day. Then it's his own data and he can grant access to everyone he wants. A GRANT means "I trust you". If someone you trust does the thing above, it's time to think it over and then REVOKE. This mechanism does also work for update rules. In the examples of the previous section, the owner of the tables in Al's database could GRANT SELECT, INSERT, UPDATE and DELETE on the shoelace view to al. But only SELECT on shoelace_log. The rule action to write log entries will still be executed successfull. And Al could see the log entries. But he cannot create fake entries, nor could he manipulate or remove existing ones. GRANT ALL currently includes RULE permission. This means the granted user could drop the rule, do the changes and reinstall it. I think this should get changed quickly. Many things that can be done using triggers can also be implemented using the Postgres rule system. What currently cannot be implemented by rules are some kinds of constraints. It is possible, to place a qualified rule that rewrites a query to NOTHING if the value of a column does not appear in another table. But then the data is silently thrown away and that's not a good idea. If checks for valid values are required, and in the case of an invalid value an error message should be generated, it must be done by a trigger for now. On the other hand a trigger that is fired on INSERT on a view can do the same as a rule, put the data somewhere else and suppress the insert in the view. But it cannot do the same thing on UPDATE or DELETE, because there is no real data in the view relation that could be scanned and thus the trigger would never get called. Only a rule will help. For the things that can be implemented by both, it depends on the usage of the database, which is the best. A trigger is fired for any row affected once. A rule manipulates the parsetree or generates an additional one. So if many rows are affected in one statement, a rule issuing one extra query would usually do a better job than a trigger that is called for any single row and must execute his operations this many times. For example: There are two tables CREATE TABLE computer ( hostname text -- indexed manufacturer text -- indexed ); CREATE TABLE software ( software text, -- indexed hostname text -- indexed ); Both tables have many thousands of rows and the index on hostname is unique. The hostname column contains the full qualified domain name of the computer. The rule/trigger should constraint delete rows from software that reference the deleted host. Since the trigger is called for each individual row deleted from computer, it can use the statement DELETE FROM software WHERE hostname = $1; in a prepared and saved plan and pass the hostname in the parameter. The rule would be written as CREATE RULE computer_del AS ON DELETE TO computer DO DELETE FROM software WHERE hostname = OLD.hostname; Now we look at different types of deletes. In the case of a DELETE FROM computer WHERE hostname = 'mypc.local.net'; the table computer is scanned by index (fast) and the query issued by the trigger would also be an index scan (fast too). The extra query from the rule would be a DELETE FROM software WHERE computer.hostname = 'mypc.local.net' AND software.hostname = computer.hostname; Since there are appropriate indices setup, the optimizer will create a plan of Nestloop -> Index Scan using comp_hostidx on computer -> Index Scan using soft_hostidx on software So there would be not that much difference in speed between the trigger and the rule implementation. With the next delete we want to get rid of all the 2000 computers where the hostname starts with 'old'. There are two possible queries to do that. One is DELETE FROM computer WHERE hostname >= 'old' AND hostname < 'ole' Where the plan for the rule query will be a Hash Join -> Seq Scan on software -> Hash -> Index Scan using comp_hostidx on computer The other possible query is a DELETE FROM computer WHERE hostname ~ '^old'; with the execution plan Nestloop -> Index Scan using comp_hostidx on computer -> Index Scan using soft_hostidx on software This shows, that the optimizer does not realize that the qualification for the hostname on computer could also be used for an index scan on software when there are multiple qualification expressions combined with AND, what he does in the regexp version of the query. The trigger will get invoked once for any of the 2000 old computers that have to be deleted and that will result in one index scan over computer and 2000 index scans for the software. The rule implementation will do it with two queries over indices. And it depends on the overall size of the software table if the rule will still be faster in the seqscan situation. 2000 query executions over the SPI manager take some time, even if all the index blocks to look them up will soon appear in the cache. The last query we look at is a DELETE FROM computer WHERE manufacurer = 'bim'; Again this could result in many rows to be deleted from computer. So the trigger will again fire many queries into the executor. But the rule plan will again be the Nestloop over two IndexScan's. Only using another index on computer: Nestloop -> Index Scan using comp_manufidx on computer -> Index Scan using soft_hostidx on software resulting from the rules query DELETE FROM software WHERE computer.manufacurer = 'bim' AND software.hostname = computer.hostname; In any of these cases, the extra queries from the rule system will be more or less independent from the number of affected rows in a query. Another situation is cases on UPDATE where it depends on the change of an attribute if an action should be performed or not. In Postgres version 6.4, the attribute specification for rule events is disabled (it will have it's comeback latest in 6.5, maybe earlier - stay tuned). So for now the only way to create a rule as in the shoelace_log example is to do it with a rule qualification. That results in an extra query that is performed always, even if the attribute of interest cannot change at all because it does not appear in the targetlist of the initial query. When this is enabled again, it will be one more advantage of rules over triggers. Optimization of a trigger must fail by definition in this case, because the fact that it's actions will only be done when a specific attribute is updated is hidden in it's functionality. The definition of a trigger only allows to specify it on row level, so whenever a row is touched, the trigger must be called to make it's decision. The rule system will know it by looking up the targetlist and will suppress the additional query completely if the attribute isn't touched. So the rule, qualified or not, will only do its scans if there ever could be something to do. Rules will only be significant slower than triggers if their actions result in large and bad qualified joins, a situation where the optimizer fails. They are a big hammer. Using a big hammer without caution can cause big damage. But used with the right touch, they can hit any nail on the head.