As it turns out, part of defining a new type is the definition of functions that describe its behavior. Consequently, while it is possible to define a new function without defining a new type, the reverse is not true. We therefore describe how to add new functions to Postgres before describing how to add new types. Postgres SQL provides three types of functions: query language functions (functions written in SQL) procedural language functions (functions written in, for example, PLTCL or PLSQL) programming language functions (functions written in a compiled programming language such as C) Every kind of function can take a base type, a composite type or some combination as arguments (parameters). In addition, every kind of function can return a base type or a composite type. It's easiest to define SQL functions, so we'll start with those. Examples in this section can also be found in funcs.sql and funcs.c. SQL functions execute an arbitrary list of SQL queries, returning the results of the last query in the list. SQL functions in general return sets. If their returntype is not specified as a setof, then an arbitrary element of the last query's result will be returned. The body of a SQL function following AS should be a list of queries separated by semicolons and bracketed within single-quote marks. Note that quote marks used in the queries must be escaped, by preceding them with a backslash. Arguments to the SQL function may be referenced in the queries using a $n syntax: $1 refers to the first argument, $2 to the second, and so on. If an argument is complex, then a dot notation (e.g. "$1.emp") may be used to access attributes of the argument or to invoke functions. To illustrate a simple SQL function, consider the following, which might be used to debit a bank account: CREATE FUNCTION tp1 (int4, float8) RETURNS int4 AS 'UPDATE bank SET balance = bank.balance - $2 WHERE bank.acctountno = $1; SELECT 1;' LANGUAGE 'sql'; A user could execute this function to debit account 17 by $100.00 as follows: SELECT tp1( 17,100.0); The following more interesting example takes a single argument of type EMP, and retrieves multiple results: CREATE FUNCTION hobbies (EMP) RETURNS SETOF hobbies AS 'SELECT hobbies.* FROM hobbies WHERE $1.name = hobbies.person' LANGUAGE 'sql'; The simplest possible SQL function has no arguments and simply returns a base type, such as int4: CREATE FUNCTION one() RETURNS int4 AS 'SELECT 1 as RESULT;' LANGUAGE 'sql'; SELECT one() AS answer; +-------+ |answer | +-------+ |1 | +-------+ Notice that we defined a column name for the function's result (with the name RESULT), but this column name is not visible outside the function. Hence, the result is labelled answer instead of one. It's almost as easy to define SQL functions that take base types as arguments. In the example below, notice how we refer to the arguments within the function as $1 and $2: CREATE FUNCTION add_em(int4, int4) RETURNS int4 AS 'SELECT $1 + $2;' LANGUAGE 'sql'; SELECT add_em(1, 2) AS answer; +-------+ |answer | +-------+ |3 | +-------+ When specifying functions with arguments of composite types (such as EMP), we must not only specify which argument we want (as we did above with $1 and $2) but also the attributes of that argument. For example, take the function double_salary that computes what your salary would be if it were doubled: CREATE FUNCTION double_salary(EMP) RETURNS int4 AS 'SELECT $1.salary * 2 AS salary;' LANGUAGE 'sql'; SELECT name, double_salary(EMP) AS dream FROM EMP WHERE EMP.cubicle ~= point '(2,1)'; +-----+-------+ |name | dream | +-----+-------+ |Sam | 2400 | +-----+-------+ Notice the use of the syntax $1.salary. Before launching into the subject of functions that return composite types, we must first introduce the function notation for projecting attributes. The simple way to explain this is that we can usually use the notations attribute(table) and table.attribute interchangably: -- -- this is the same as: -- SELECT EMP.name AS youngster FROM EMP WHERE EMP.age < 30 -- SELECT name(EMP) AS youngster FROM EMP WHERE age(EMP) < 30; +----------+ |youngster | +----------+ |Sam | +----------+ As we shall see, however, this is not always the case. This function notation is important when we want to use a function that returns a single row. We do this by assembling the entire row within the function, attribute by attribute. This is an example of a function that returns a single EMP row: CREATE FUNCTION new_emp() RETURNS EMP AS 'SELECT text ''None'' AS name, 1000 AS salary, 25 AS age, point ''(2,2)'' AS cubicle' LANGUAGE 'sql'; In this case we have specified each of the attributes with a constant value, but any computation or expression could have been substituted for these constants. Defining a function like this can be tricky. Some of the more important caveats are as follows: The target list order must be exactly the same as that in which the attributes appear in the CREATE TABLE statement that defined the composite type. You must typecast the expressions to match the composite type's definition, or you will get errors like this: ERROR: function declared to return emp returns varchar instead of text at column 1 When calling a function that returns a row, we cannot retrieve the entire row. We must either project an attribute out of the row or pass the entire row into another function. SELECT name(new_emp()) AS nobody; +-------+ |nobody | +-------+ |None | +-------+ The reason why, in general, we must use the function syntax for projecting attributes of function return values is that the parser just doesn't understand the other (dot) syntax for projection when combined with function calls. SELECT new_emp().name AS nobody; NOTICE:parser: syntax error at or near "." Any collection of commands in the SQL query language can be packaged together and defined as a function. The commands can include updates (i.e., INSERT, UPDATE, and DELETE) as well as SELECT queries. However, the final command must be a SELECT that returns whatever is specified as the function's returntype. CREATE FUNCTION clean_EMP () RETURNS int4 AS 'DELETE FROM EMP WHERE EMP.salary <= 0; SELECT 1 AS ignore_this;' LANGUAGE 'sql'; SELECT clean_EMP(); +--+ |x | +--+ |1 | +--+ Procedural languages aren't built into Postgres. They are offered by loadable modules. Please refer to the documentation for the PL in question for details about the syntax and how the AS clause is interpreted by the PL handler. There are currently three procedural languages available in the standard Postgres distribution (PLSQL, PLTCL and PLPERL), and other languages can be defined. Refer to for more information. Internal functions are functions written in C that have been statically linked into the Postgres backend process. The AS clause gives the C-language name of the function, which need not be the same as the name being declared for SQL use. (For reasons of backwards compatibility, an empty AS string is accepted as meaning that the C-language function name is the same as the SQL name.) Normally, all internal functions present in the backend are declared as SQL functions during database initialization, but a user could use CREATE FUNCTION to create additional alias names for an internal function. Internal functions are declared in CREATE FUNCTION with language name internal. Functions written in C can be compiled into dynamically loadable objects (also called shared libraries), and used to implement user-defined SQL functions. The first time a user-defined function in a particular loadable object file is called in a backend session, the dynamic loader loads that object file into memory so that the function can be called. The CREATE FUNCTION for a user-defined function must therefore specify two pieces of information for the function: the name of the loadable object file, and the C name (link symbol) of the specific function to call within that object file. If the C name is not explicitly specified then it is assumed to be the same as the SQL function name. After it is used for the first time, a dynamically loaded user function is retained in memory, and future calls to the function in the same session will only incur the small overhead of a symbol table lookup. The string that specifies the object file (the first string in the AS clause) should be the full path of the object code file for the function, bracketed by single quote marks. If a link symbol is given in the AS clause, the link symbol should also be bracketed by single quote marks, and should be exactly the same as the name of the function in the C source code. On Unix systems the command nm will print all of the link symbols in a dynamically loadable object. Postgres will not compile a function automatically; it must be compiled before it is used in a CREATE FUNCTION command. See below for additional information. Two different calling conventions are currently used for C functions. The newer "version 1" calling convention is indicated by writing a PG_FUNCTION_INFO_V1() macro call for the function, as illustrated below. Lack of such a macro indicates an old-style ("version 0") function. The language name specified in CREATE FUNCTION is 'C' in either case. Old-style functions are now deprecated because of portability problems and lack of functionality, but they are still supported for compatibility reasons. The following table gives the C type required for parameters in the C functions that will be loaded into Postgres. The "Defined In" column gives the actual header file (in the .../src/backend/ directory) that the equivalent C type is defined. However, if you include utils/builtins.h, these files will automatically be included. Equivalent C Types Built-In Type C Type Defined In abstime AbsoluteTime utils/nabstime.h bool bool include/c.h box (BOX *) utils/geo-decls.h bytea (bytea *) include/postgres.h "char" char N/A cid CID include/postgres.h datetime (DateTime *) include/c.h or include/postgres.h int2 int2 or int16 include/postgres.h int2vector (int2vector *) include/postgres.h int4 int4 or int32 include/postgres.h float4 (float4 *) include/c.h or include/postgres.h float8 (float8 *) include/c.h or include/postgres.h lseg (LSEG *) include/geo-decls.h name (Name) include/postgres.h oid oid include/postgres.h oidvector (oidvector *) include/postgres.h path (PATH *) utils/geo-decls.h point (POINT *) utils/geo-decls.h regproc regproc or REGPROC include/postgres.h reltime RelativeTime utils/nabstime.h text (text *) include/postgres.h tid ItemPointer storage/itemptr.h timespan (TimeSpan *) include/c.h or include/postgres.h tinterval TimeInterval utils/nabstime.h xid (XID *) include/postgres.h Internally, Postgres regards a base type as a "blob of memory." The user-defined functions that you define over a type in turn define the way that Postgres can operate on it. That is, Postgres will only store and retrieve the data from disk and use your user-defined functions to input, process, and output the data. Base types can have one of three internal formats: pass by value, fixed-length pass by reference, fixed-length pass by reference, variable-length By-value types can only be 1, 2 or 4 bytes in length (even if your computer supports by-value types of other sizes). Postgres itself only passes integer types by value. You should be careful to define your types such that they will be the same size (in bytes) on all architectures. For example, the long type is dangerous because it is 4 bytes on some machines and 8 bytes on others, whereas int type is 4 bytes on most Unix machines (though not on most personal computers). A reasonable implementation of the int4 type on Unix machines might be: /* 4-byte integer, passed by value */ typedef int int4; On the other hand, fixed-length types of any size may be passed by-reference. For example, here is a sample implementation of a Postgres type: /* 16-byte structure, passed by reference */ typedef struct { double x, y; } Point; Only pointers to such types can be used when passing them in and out of Postgres functions. To return a value of such a type, allocate the right amount of memory with palloc(), fill in the allocated memory, and return a pointer to it. Finally, all variable-length types must also be passed by reference. All variable-length types must begin with a length field of exactly 4 bytes, and all data to be stored within that type must be located in the memory immediately following that length field. The length field is the total length of the structure (i.e., it includes the size of the length field itself). We can define the text type as follows: typedef struct { int4 length; char data[1]; } text; Obviously, the data field shown here is not long enough to hold all possible strings; it's impossible to declare such a structure in C. When manipulating variable-length types, we must be careful to allocate the correct amount of memory and initialize the length field. For example, if we wanted to store 40 bytes in a text structure, we might use a code fragment like this: #include "postgres.h" ... char buffer[40]; /* our source data */ ... text *destination = (text *) palloc(VARHDRSZ + 40); destination->length = VARHDRSZ + 40; memmove(destination->data, buffer, 40); ... Now that we've gone over all of the possible structures for base types, we can show some examples of real functions. We present the old style calling convention first --- although this approach is now deprecated, it's easier to get a handle on initially. In the version-0 method, the arguments and result of the C function are just declared in normal C style, but being careful to use the C representation of each SQL data type as shown above. Here are some examples: #include <string.h> #include "postgres.h" /* By Value */ int add_one(int arg) { return arg + 1; } /* By Reference, Fixed Length */ float8 * add_one_float8(float8 *arg) { float8 *result = (float8 *) palloc(sizeof(float8)); *result = *arg + 1.0; return result; } Point * makepoint(Point *pointx, Point *pointy) { Point *new_point = (Point *) palloc(sizeof(Point)); new_point->x = pointx->x; new_point->y = pointy->y; return new_point; } /* By Reference, Variable Length */ text * copytext(text *t) { /* * VARSIZE is the total size of the struct in bytes. */ text *new_t = (text *) palloc(VARSIZE(t)); VARATT_SIZEP(new_t) = VARSIZE(t); /* * VARDATA is a pointer to the data region of the struct. */ memcpy((void *) VARDATA(new_t), /* destination */ (void *) VARDATA(t), /* source */ VARSIZE(t)-VARHDRSZ); /* how many bytes */ return new_t; } text * concat_text(text *arg1, text *arg2) { int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ; text *new_text = (text *) palloc(new_text_size); memset((void *) new_text, 0, new_text_size); VARATT_SIZEP(new_text) = new_text_size; strncpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ); strncat(VARDATA(new_text), VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ); return new_text; } Supposing that the above code has been prepared in file funcs.c and compiled into a shared object, we could define the functions to Postgres with commands like this: CREATE FUNCTION add_one(int4) RETURNS int4 AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c' WITH (isStrict); -- note overloading of SQL function name add_one() CREATE FUNCTION add_one(float8) RETURNS float8 AS 'PGROOT/tutorial/funcs.so', 'add_one_float8' LANGUAGE 'c' WITH (isStrict); CREATE FUNCTION makepoint(point, point) RETURNS point AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c' WITH (isStrict); CREATE FUNCTION copytext(text) RETURNS text AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c' WITH (isStrict); CREATE FUNCTION concat_text(text, text) RETURNS text AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c' WITH (isStrict); Here PGROOT stands for the full path to the Postgres source tree. Note that depending on your system, the filename for a shared object might not end in .so, but in .sl or something else; adapt accordingly. Notice that we have specified the functions as "strict", meaning that the system should automatically assume a NULL result if any input value is NULL. By doing this, we avoid having to check for NULL inputs in the function code. Without this, we'd have to check for NULLs explicitly, for example by checking for a null pointer for each pass-by-reference argument. (For pass-by-value arguments, we don't even have a way to check!) Although this calling convention is simple to use, it is not very portable; on some architectures there are problems with passing smaller-than-int data types this way. Also, there is no simple way to return a NULL result, nor to cope with NULL arguments in any way other than making the function strict. The version-1 convention, presented next, overcomes these objections. The version-1 calling convention relies on macros to suppress most of the complexity of passing arguments and results. The C declaration of a version-1 function is always Datum funcname(PG_FUNCTION_ARGS) In addition, the macro call PG_FUNCTION_INFO_V1(funcname); must appear in the same source file (conventionally it's written just before the function itself). This macro call is not needed for "internal"-language functions, since Postgres currently assumes all internal functions are version-1. However, it is required for dynamically-loaded functions. In a version-1 function, each actual argument is fetched using a PG_GETARG_xxx() macro that corresponds to the argument's datatype, and the result is returned using a PG_RETURN_xxx() macro for the return type. Here we show the same functions as above, coded in version-1 style: #include <string.h> #include "postgres.h" #include "fmgr.h" /* By Value */ PG_FUNCTION_INFO_V1(add_one); Datum add_one(PG_FUNCTION_ARGS) { int32 arg = PG_GETARG_INT32(0); PG_RETURN_INT32(arg + 1); } /* By Reference, Fixed Length */ PG_FUNCTION_INFO_V1(add_one_float8); Datum add_one_float8(PG_FUNCTION_ARGS) { /* The macros for FLOAT8 hide its pass-by-reference nature */ float8 arg = PG_GETARG_FLOAT8(0); PG_RETURN_FLOAT8(arg + 1.0); } PG_FUNCTION_INFO_V1(makepoint); Datum makepoint(PG_FUNCTION_ARGS) { /* Here, the pass-by-reference nature of Point is not hidden */ Point *pointx = PG_GETARG_POINT_P(0); Point *pointy = PG_GETARG_POINT_P(1); Point *new_point = (Point *) palloc(sizeof(Point)); new_point->x = pointx->x; new_point->y = pointy->y; PG_RETURN_POINT_P(new_point); } /* By Reference, Variable Length */ PG_FUNCTION_INFO_V1(copytext); Datum copytext(PG_FUNCTION_ARGS) { text *t = PG_GETARG_TEXT_P(0); /* * VARSIZE is the total size of the struct in bytes. */ text *new_t = (text *) palloc(VARSIZE(t)); VARATT_SIZEP(new_t) = VARSIZE(t); /* * VARDATA is a pointer to the data region of the struct. */ memcpy((void *) VARDATA(new_t), /* destination */ (void *) VARDATA(t), /* source */ VARSIZE(t)-VARHDRSZ); /* how many bytes */ PG_RETURN_TEXT_P(new_t); } PG_FUNCTION_INFO_V1(concat_text); Datum concat_text(PG_FUNCTION_ARGS) { text *arg1 = PG_GETARG_TEXT_P(0); text *arg2 = PG_GETARG_TEXT_P(1); int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ; text *new_text = (text *) palloc(new_text_size); memset((void *) new_text, 0, new_text_size); VARATT_SIZEP(new_text) = new_text_size; strncpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ); strncat(VARDATA(new_text), VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ); PG_RETURN_TEXT_P(new_text); } The CREATE FUNCTION commands are the same as for the version-0 equivalents. At first glance, the version-1 coding conventions may appear to be just pointless obscurantism. However, they do offer a number of improvements, because the macros can hide unnecessary detail. An example is that in coding add_one_float8, we no longer need to be aware that float8 is a pass-by-reference type. Another example is that the GETARG macros for variable-length types hide the need to deal with fetching "toasted" (compressed or out-of-line) values. The old-style copytext and concat_text functions shown above are actually wrong in the presence of toasted values, because they don't call pg_detoast_datum() on their inputs. (The handler for old-style dynamically-loaded functions currently takes care of this detail, but it does so less efficiently than is possible for a version-1 function.) The version-1 function call conventions also make it possible to test for NULL inputs to a non-strict function, return a NULL result (from either strict or non-strict functions), return set results, and implement trigger functions and procedural-language call handlers. Version-1 code is also more portable than version-0, because it does not break ANSI C restrictions on function call protocol. For more details see src/backend/utils/fmgr/README in the source distribution. Composite types do not have a fixed layout like C structures. Instances of a composite type may contain null fields. In addition, composite types that are part of an inheritance hierarchy may have different fields than other members of the same inheritance hierarchy. Therefore, Postgres provides a procedural interface for accessing fields of composite types from C. As Postgres processes a set of rows, each row will be passed into your function as an opaque structure of type TUPLE. Suppose we want to write a function to answer the query SELECT name, c_overpaid(emp, 1500) AS overpaid FROM emp WHERE name = 'Bill' OR name = 'Sam'; In the query above, we can define c_overpaid as: #include "postgres.h" #include "executor/executor.h" /* for GetAttributeByName() */ bool c_overpaid(TupleTableSlot *t, /* the current row of EMP */ int32 limit) { bool isnull; int32 salary; salary = DatumGetInt32(GetAttributeByName(t, "salary", &isnull)); if (isnull) return (false); return salary > limit; } /* In version-1 coding, the above would look like this: */ PG_FUNCTION_INFO_V1(c_overpaid); Datum c_overpaid(PG_FUNCTION_ARGS) { TupleTableSlot *t = (TupleTableSlot *) PG_GETARG_POINTER(0); int32 limit = PG_GETARG_INT32(1); bool isnull; int32 salary; salary = DatumGetInt32(GetAttributeByName(t, "salary", &isnull)); if (isnull) PG_RETURN_BOOL(false); /* Alternatively, we might prefer to do PG_RETURN_NULL() for null salary */ PG_RETURN_BOOL(salary > limit); } GetAttributeByName is the Postgres system function that returns attributes out of the current row. It has three arguments: the argument of type TupleTableSlot* passed into the function, the name of the desired attribute, and a return parameter that tells whether the attribute is null. GetAttributeByName returns a Datum value that you can convert to the proper datatype by using the appropriate DatumGetXXX() macro. The following query lets Postgres know about the c_overpaid function: CREATE FUNCTION c_overpaid(emp, int4) RETURNS bool AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c'; While there are ways to construct new rows or modify existing rows from within a C function, these are far too complex to discuss in this manual. We now turn to the more difficult task of writing programming language functions. Be warned: this section of the manual will not make you a programmer. You must have a good understanding of C (including the use of pointers and the malloc memory manager) before trying to write C functions for use with Postgres. While it may be possible to load functions written in languages other than C into Postgres, this is often difficult (when it is possible at all) because other languages, such as FORTRAN and Pascal often do not follow the same calling convention as C. That is, other languages do not pass argument and return values between functions in the same way. For this reason, we will assume that your programming language functions are written in C. The basic rules for building C functions are as follows: The relevant header (include) files are installed under /usr/local/pgsql/include or equivalent. You can use pg_config --includedir to find out where it is on your system (or the system that your users will be running on). For very low-level work you might need to have a complete PostgreSQL source tree available. When allocating memory, use the Postgres routines palloc and pfree instead of the corresponding C library routines malloc and free. The memory allocated by palloc will be freed automatically at the end of each transaction, preventing memory leaks. Always zero the bytes of your structures using memset or bzero. Several routines (such as the hash access method, hash join and the sort algorithm) compute functions of the raw bits contained in your structure. Even if you initialize all fields of your structure, there may be several bytes of alignment padding (holes in the structure) that may contain garbage values. Most of the internal Postgres types are declared in postgres.h, the function manager interfaces (PG_FUNCTION_ARGS, etc.) are in fmgr.h, so you will need to include at least these two files. Including postgres.h will also include elog.h and palloc.h for you. Symbol names defined within object files must not conflict with each other or with symbols defined in the PostgreSQL server executable. You will have to rename your functions or variables if you get error messages to this effect. Compiling and linking your object code so that it can be dynamically loaded into Postgres always requires special flags. See for a detailed explanation of how to do it for your particular operating system. &dfunc; More than one function may be defined with the same name, as long as the arguments they take are different. In other words, function names can be overloaded. A function may also have the same name as an attribute. In the case that there is an ambiguity between a function on a complex type and an attribute of the complex type, the attribute will always be used. As of Postgres 7.0, the alternative form of the AS clause for the SQL CREATE FUNCTION command decouples the SQL function name from the function name in the C source code. This is now the preferred technique to accomplish function overloading. For functions written in C, the SQL name declared in CREATE FUNCTION must be exactly the same as the actual name of the function in the C code (hence it must be a legal C function name). There is a subtle implication of this restriction: while the dynamic loading routines in most operating systems are more than happy to allow you to load any number of shared libraries that contain conflicting (identically-named) function names, they may in fact botch the load in interesting ways. For example, if you define a dynamically-loaded function that happens to have the same name as a function built into Postgres, the DEC OSF/1 dynamic loader causes Postgres to call the function within itself rather than allowing Postgres to call your function. Hence, if you want your function to be used on different architectures, we recommend that you do not overload C function names. There is a clever trick to get around the problem just described. Since there is no problem overloading SQL functions, you can define a set of C functions with different names and then define a set of identically-named SQL function wrappers that take the appropriate argument types and call the matching C function. Another solution is not to use dynamic loading, but to link your functions into the backend statically and declare them as INTERNAL functions. Then, the functions must all have distinct C names but they can be declared with the same SQL names (as long as their argument types differ, of course). This way avoids the overhead of an SQL wrapper function, at the cost of more effort to prepare a custom backend executable. (This option is only available in version 6.5 and later, since prior versions required internal functions to have the same name in SQL as in the C code.)