Merge query of system objects FAQ items.
This commit is contained in:
Bruce Momjian
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doc/FAQ
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@ -1,7 +1,7 @@
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Sun Jan 9 14:44:04 EST 2005
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Last updated: Sat Jan 15 00:18:38 EST 2005
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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@ -60,43 +60,42 @@
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4.1) What is the difference between binary cursors and normal cursors?
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4.2) How do I SELECT only the first few rows of a query? A random row?
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4.3) How do I get a list of tables or other things I can see in psql?
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4.3) How do I find out what tables, indexes, databases, and users are
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defined? How do I see the queries used by psql to display them?
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4.4) How do you remove a column from a table, or change it's data
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type?
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4.5) What is the maximum size for a row, a table, and a database?
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4.6) How much database disk space is required to store data from a
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typical text file?
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4.7) How do I find out what tables, indexes, databases, and users are
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defined?
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4.8) My queries are slow or don't make use of the indexes. Why?
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4.9) How do I see how the query optimizer is evaluating my query?
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4.10) What is an R-tree index?
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4.11) What is the Genetic Query Optimizer?
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4.12) How do I perform regular expression searches and
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4.7) My queries are slow or don't make use of the indexes. Why?
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4.8) How do I see how the query optimizer is evaluating my query?
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4.9) What is an R-tree index?
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4.10) What is the Genetic Query Optimizer?
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4.11) How do I perform regular expression searches and
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case-insensitive regular expression searches? How do I use an index
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for case-insensitive searches?
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4.13) In a query, how do I detect if a field is NULL?
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4.14) What is the difference between the various character types?
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4.15.1) How do I create a serial/auto-incrementing field?
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4.15.2) How do I get the value of a SERIAL insert?
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4.15.3) Doesn't currval() lead to a race condition with other users?
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4.15.4) Why aren't my sequence numbers reused on transaction abort?
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4.12) In a query, how do I detect if a field is NULL?
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4.13) What is the difference between the various character types?
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4.14.0) How do I create a serial/auto-incrementing field?
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4.14.1) How do I get the value of a SERIAL insert?
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4.14.2) Doesn't currval() lead to a race condition with other users?
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4.14.3) Why aren't my sequence numbers reused on transaction abort?
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Why are there gaps in the numbering of my sequence/SERIAL column?
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4.16) What is an OID? What is a TID?
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4.17) What is the meaning of some of the terms used in PostgreSQL?
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4.18) Why do I get the error "ERROR: Memory exhausted in
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4.15) What is an OID? What is a TID?
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4.16) What is the meaning of some of the terms used in PostgreSQL?
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4.17) Why do I get the error "ERROR: Memory exhausted in
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AllocSetAlloc()"?
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4.19) How do I tell what PostgreSQL version I am running?
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4.20) Why does my large-object operations get "invalid large obj
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4.18) How do I tell what PostgreSQL version I am running?
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4.19) Why does my large-object operations get "invalid large obj
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descriptor"?
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4.21) How do I create a column that will default to the current time?
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4.22) Why are my subqueries using IN so slow?
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4.23) How do I perform an outer join?
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4.24) How do I perform queries using multiple databases?
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4.25) How do I return multiple rows or columns from a function?
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4.26) Why can't I reliably create/drop temporary tables in PL/PgSQL
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4.20) How do I create a column that will default to the current time?
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4.21) Why are my subqueries using IN so slow?
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4.22) How do I perform an outer join?
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4.23) How do I perform queries using multiple databases?
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4.24) How do I return multiple rows or columns from a function?
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4.25) Why can't I reliably create/drop temporary tables in PL/PgSQL
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functions?
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4.27) What encryption options are available?
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4.26) What encryption options are available?
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Extending PostgreSQL
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@ -659,7 +658,8 @@ log_*
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ORDER BY random()
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LIMIT 1;
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4.3) How do I get a list of tables or other things I can see in psql?
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4.3) How do I find out what tables, indexes, databases, and users are
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defined? How do I see the queries used by psql to display them?
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Use the \dt command to see tables in psql. For a complete list of
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commands inside psql you can use \?. Alternatively you can read the
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@ -667,9 +667,16 @@ log_*
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contains SQL commands that generate the output for psql's backslash
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commands. You can also start psql with the -E option so it will print
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out the queries it uses to execute the commands you give. PostgreSQL
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also provides an SQLi compliant INFORMATION SCHEMA interface you can
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also provides an SQL compliant INFORMATION SCHEMA interface you can
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query to get information about the database.
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There are also system tables beginning with pg_ that describe these
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too. Use psql -l will list all databases.
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Also try the file pgsql/src/tutorial/syscat.source. It illustrates
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many of the SELECTs needed to get information from the database system
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tables.
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4.4) How do you remove a column from a table, or change its data type?
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DROP COLUMN functionality was added in release 7.3 with ALTER TABLE
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@ -749,18 +756,7 @@ log_*
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NULLs are stored as bitmaps, so they use very little space.
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4.7) How do I find out what tables, indexes, databases, and users are
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defined?
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psql has a variety of backslash commands to show such information. Use
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\? to see them. There are also system tables beginning with pg_ that
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describe these too. Also, psql -l will list all databases.
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Also try the file pgsql/src/tutorial/syscat.source. It illustrates
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many of the SELECTs needed to get information from the database system
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tables.
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4.8) My queries are slow or don't make use of the indexes. Why?
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4.7) My queries are slow or don't make use of the indexes. Why?
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Indexes are not automatically used by every query. Indexes are only
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used if the table is larger than a minimum size, and the query selects
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@ -802,7 +798,7 @@ log_*
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[a-e].
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* Case-insensitive searches such as ILIKE and ~* do not utilise
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indexes. Instead, use functional indexes, which are described in
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section 4.12.
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section 4.11.
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* The default C locale must be used during initdb because it is not
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possible to know the next-greater character in a non-C locale. You
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can create a special
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@ -815,11 +811,11 @@ LIKE
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types exactly match the index's column types. This is particularly
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true of int2, int8, and numeric column indexes.
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4.9) How do I see how the query optimizer is evaluating my query?
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4.8) How do I see how the query optimizer is evaluating my query?
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See the EXPLAIN manual page.
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4.10) What is an R-tree index?
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4.9) What is an R-tree index?
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An R-tree index is used for indexing spatial data. A hash index can't
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handle range searches. A B-tree index only handles range searches in a
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@ -842,13 +838,13 @@ LIKE
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extending R-trees requires a bit of work and we don't currently have
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any documentation on how to do it.
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4.11) What is the Genetic Query Optimizer?
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4.10) What is the Genetic Query Optimizer?
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The GEQO module speeds query optimization when joining many tables by
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means of a Genetic Algorithm (GA). It allows the handling of large
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join queries through nonexhaustive search.
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4.12) How do I perform regular expression searches and case-insensitive
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4.11) How do I perform regular expression searches and case-insensitive
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regular expression searches? How do I use an index for case-insensitive
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searches?
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@ -865,11 +861,11 @@ LIKE
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functional index, it will be used:
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CREATE INDEX tabindex ON tab (lower(col));
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4.13) In a query, how do I detect if a field is NULL?
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4.12) In a query, how do I detect if a field is NULL?
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You test the column with IS NULL and IS NOT NULL.
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4.14) What is the difference between the various character types?
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4.13) What is the difference between the various character types?
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Type Internal Name Notes
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--------------------------------------------------
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@ -897,7 +893,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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particularly values that include NULL bytes. All the types described
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here have similar performance characteristics.
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4.15.1) How do I create a serial/auto-incrementing field?
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4.14.1) How do I create a serial/auto-incrementing field?
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PostgreSQL supports a SERIAL data type. It auto-creates a sequence.
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For example, this:
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@ -918,11 +914,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
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However, if you need to dump and reload the database, you need to use
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pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
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4.15.2) How do I get the value of a SERIAL insert?
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4.14.2) How do I get the value of a SERIAL insert?
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One approach is to retrieve the next SERIAL value from the sequence
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object with the nextval() function before inserting and then insert it
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explicitly. Using the example table in 4.15.1, an example in a
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explicitly. Using the example table in 4.14.1, an example in a
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pseudo-language would look like this:
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new_id = execute("SELECT nextval('person_id_seq')");
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execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
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@ -944,19 +940,19 @@ BYTEA bytea variable-length byte array (null-byte safe)
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billion. In Perl, using DBI with the DBD::Pg module, the oid value is
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made available via $sth->{pg_oid_status} after $sth->execute().
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4.15.3) Doesn't currval() lead to a race condition with other users?
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4.14.3) Doesn't currval() lead to a race condition with other users?
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No. currval() returns the current value assigned by your backend, not
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by all users.
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4.15.4) Why aren't my sequence numbers reused on transaction abort? Why are
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4.14.4) Why aren't my sequence numbers reused on transaction abort? Why are
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there gaps in the numbering of my sequence/SERIAL column?
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To improve concurrency, sequence values are given out to running
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transactions as needed and are not locked until the transaction
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completes. This causes gaps in numbering from aborted transactions.
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4.16) What is an OID? What is a TID?
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4.15) What is an OID? What is a TID?
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OIDs are PostgreSQL's answer to unique row ids. Every row that is
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created in PostgreSQL gets a unique OID. All OIDs generated during
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@ -989,7 +985,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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values. TIDs change after rows are modified or reloaded. They are used
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by index entries to point to physical rows.
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4.17) What is the meaning of some of the terms used in PostgreSQL?
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4.16) What is the meaning of some of the terms used in PostgreSQL?
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Some of the source code and older documentation use terms that have
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more common usage. Here are some:
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@ -1007,7 +1003,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
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/glossary.html
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4.18) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
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4.17) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
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You probably have run out of virtual memory on your system, or your
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kernel has a low limit for certain resources. Try this before starting
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@ -1022,11 +1018,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
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problem with the SQL client because the backend is returning too much
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data, try it before starting the client.
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4.19) How do I tell what PostgreSQL version I am running?
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4.18) How do I tell what PostgreSQL version I am running?
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From psql, type SELECT version();
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4.20) Why does my large-object operations get "invalid large obj
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4.19) Why does my large-object operations get "invalid large obj
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descriptor"?
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You need to put BEGIN WORK and COMMIT around any use of a large object
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@ -1041,12 +1037,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
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If you are using a client interface like ODBC you may need to set
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auto-commit off.
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4.21) How do I create a column that will default to the current time?
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4.20) How do I create a column that will default to the current time?
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Use CURRENT_TIMESTAMP:
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CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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4.22) Why are my subqueries using IN so slow?
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4.21) Why are my subqueries using IN so slow?
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In versions prior to 7.4, subqueries were joined to outer queries by
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sequentially scanning the result of the subquery for each row of the
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@ -1067,7 +1063,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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In version 7.4 and later, IN actually uses the same sophisticated join
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techniques as normal queries, and is prefered to using EXISTS.
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4.23) How do I perform an outer join?
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4.22) How do I perform an outer join?
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PostgreSQL supports outer joins using the SQL standard syntax. Here
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are two examples:
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@ -1097,7 +1093,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
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ORDER BY col1
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4.24) How do I perform queries using multiple databases?
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4.23) How do I perform queries using multiple databases?
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There is no way to query a database other than the current one.
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Because PostgreSQL loads database-specific system catalogs, it is
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@ -1107,12 +1103,12 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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course, a client can make simultaneous connections to different
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databases and merge the results on the client side.
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4.25) How do I return multiple rows or columns from a function?
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4.24) How do I return multiple rows or columns from a function?
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In 7.3, you can easily return multiple rows or columns from a
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function, http://techdocs.postgresql.org/guides/SetReturningFunctions.
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4.26) Why can't I reliably create/drop temporary tables in PL/PgSQL
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4.25) Why can't I reliably create/drop temporary tables in PL/PgSQL
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functions?
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PL/PgSQL caches function contents, and an unfortunate side effect is
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@ -1123,7 +1119,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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table access in PL/PgSQL. This will cause the query to be reparsed
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every time.
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4.27) What encryption options are available?
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4.26) What encryption options are available?
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* contrib/pgcrypto contains many encryption functions for use in SQL
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queries.
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@ -10,7 +10,7 @@
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alink="#0000ff">
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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
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<P>Last updated: Sun Jan 9 14:44:04 EST 2005</P>
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<P>Last updated: Sat Jan 15 00:18:38 EST 2005</P>
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<P>Current maintainer: Bruce Momjian (<A href=
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"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
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@ -86,60 +86,59 @@
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cursors and normal cursors?<BR>
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<A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
|
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first few rows of a query? A random row?<BR>
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<A href="#4.3">4.3</A>) How do I get a list of tables or other
|
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things I can see in <I>psql</I>?<BR>
|
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<A href="#4.3">4.3</A>) How do I find out what tables, indexes,
|
||||
databases, and users are defined? How do I see the queries used
|
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by <I>psql</I> to display them?<BR>
|
||||
<A href="#4.4">4.4</A>) How do you remove a column from a
|
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table, or change it's data type?<BR>
|
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<A href="#4.5">4.5</A>) What is the maximum size for a row, a
|
||||
table, and a database?<BR>
|
||||
<A href="#4.6">4.6</A>) How much database disk space is required
|
||||
to store data from a typical text file?<BR>
|
||||
<A href="#4.7">4.7</A>) How do I find out what tables, indexes,
|
||||
databases, and users are defined?<BR>
|
||||
<A href="#4.8">4.8</A>) My queries are slow or don't make use of
|
||||
<A href="#4.7">4.7</A>) My queries are slow or don't make use of
|
||||
the indexes. Why?<BR>
|
||||
<A href="#4.9">4.9</A>) How do I see how the query optimizer is
|
||||
<A href="#4.8">4.8</A>) How do I see how the query optimizer is
|
||||
evaluating my query?<BR>
|
||||
<A href="#4.10">4.10</A>) What is an R-tree index?<BR>
|
||||
<A href="#4.11">4.11</A>) What is the Genetic Query Optimizer?<BR>
|
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<A href="#4.12">4.12</A>) How do I perform regular expression
|
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<A href="#4.9">4.9</A>) What is an R-tree index?<BR>
|
||||
<A href="#4.10">4.10</A>) What is the Genetic Query Optimizer?<BR>
|
||||
<A href="#4.11">4.11</A>) How do I perform regular expression
|
||||
searches and case-insensitive regular expression searches? How do I
|
||||
use an index for case-insensitive searches?<BR>
|
||||
<A href="#4.13">4.13</A>) In a query, how do I detect if a field
|
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<A href="#4.12">4.12</A>) In a query, how do I detect if a field
|
||||
is <SMALL>NULL</SMALL>?<BR>
|
||||
<A href="#4.14">4.14</A>) What is the difference between the
|
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<A href="#4.13">4.13</A>) What is the difference between the
|
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various character types?<BR>
|
||||
<A href="#4.15.1">4.15.1</A>) How do I create a
|
||||
<A href="#4.14.0">4.14.0</A>) How do I create a
|
||||
serial/auto-incrementing field?<BR>
|
||||
<A href="#4.15.2">4.15.2</A>) How do I get the value of a
|
||||
<A href="#4.14.1">4.14.1</A>) How do I get the value of a
|
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<SMALL>SERIAL</SMALL> insert?<BR>
|
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<A href="#4.15.3">4.15.3</A>) Doesn't <I>currval()</I>
|
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<A href="#4.14.2">4.14.2</A>) Doesn't <I>currval()</I>
|
||||
lead to a race condition with other users?<BR>
|
||||
<A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers
|
||||
<A href="#4.14.3">4.14.3</A>) Why aren't my sequence numbers
|
||||
reused on transaction abort? Why are there gaps in the numbering of
|
||||
my sequence/SERIAL column?<BR>
|
||||
<A href="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
|
||||
<A href="#4.15">4.15</A>) What is an <SMALL>OID</SMALL>? What is a
|
||||
<SMALL>TID</SMALL>?<BR>
|
||||
<A href="#4.17">4.17</A>) What is the meaning of some of the terms
|
||||
<A href="#4.16">4.16</A>) What is the meaning of some of the terms
|
||||
used in PostgreSQL?<BR>
|
||||
<A href="#4.18">4.18</A>) Why do I get the error <I>"ERROR: Memory
|
||||
<A href="#4.17">4.17</A>) Why do I get the error <I>"ERROR: Memory
|
||||
exhausted in AllocSetAlloc()"</I>?<BR>
|
||||
<A href="#4.19">4.19</A>) How do I tell what PostgreSQL version I
|
||||
<A href="#4.18">4.18</A>) How do I tell what PostgreSQL version I
|
||||
am running?<BR>
|
||||
<A href="#4.20">4.20</A>) Why does my large-object operations get
|
||||
<A href="#4.19">4.19</A>) Why does my large-object operations get
|
||||
<I>"invalid large obj descriptor"</I>?<BR>
|
||||
<A href="#4.21">4.21</A>) How do I create a column that will
|
||||
<A href="#4.20">4.20</A>) How do I create a column that will
|
||||
default to the current time?<BR>
|
||||
<A href="#4.22">4.22</A>) Why are my subqueries using
|
||||
<A href="#4.21">4.21</A>) Why are my subqueries using
|
||||
<CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
|
||||
<A href="#4.23">4.23</A>) How do I perform an outer join?<BR>
|
||||
<A href="#4.24">4.24</A>) How do I perform queries using multiple
|
||||
<A href="#4.22">4.22</A>) How do I perform an outer join?<BR>
|
||||
<A href="#4.23">4.23</A>) How do I perform queries using multiple
|
||||
databases?<BR>
|
||||
<A href="#4.25">4.25</A>) How do I return multiple rows or columns
|
||||
<A href="#4.24">4.24</A>) How do I return multiple rows or columns
|
||||
from a function?<BR>
|
||||
<A href="#4.26">4.26</A>) Why can't I reliably create/drop
|
||||
<A href="#4.25">4.25</A>) Why can't I reliably create/drop
|
||||
temporary tables in PL/PgSQL functions?<BR>
|
||||
<A href="#4.27">4.27</A>) What encryption options are available?<BR>
|
||||
<A href="#4.26">4.26</A>) What encryption options are available?<BR>
|
||||
|
||||
|
||||
<H2 align="center">Extending PostgreSQL</H2>
|
||||
|
|
@ -831,8 +830,9 @@
|
|||
LIMIT 1;
|
||||
</PRE>
|
||||
|
||||
<H4><A name="4.3">4.3</A>) How do I get a list of tables or other
|
||||
things I can see in <I>psql</I>?</H4>
|
||||
<H4><A name="4.3">4.3</A>) How do I find out what tables, indexes,
|
||||
databases, and users are defined? How do I see the queries used
|
||||
by <I>psql</I> to display them?</H4>
|
||||
|
||||
<P>Use the \dt command to see tables in <I>psql</I>. For a complete list of
|
||||
commands inside psql you can use \?. Alternatively you can read the source
|
||||
|
|
@ -840,10 +840,17 @@
|
|||
contains <SMALL>SQL</SMALL> commands that generate the output for
|
||||
<I>psql</I>'s backslash commands. You can also start <I>psql</I> with the
|
||||
<I>-E</I> option so it will print out the queries it uses to execute the
|
||||
commands you give. PostgreSQL also provides an <SMALL>SQLi</SMALL> compliant
|
||||
commands you give. PostgreSQL also provides an <SMALL>SQL</SMALL> compliant
|
||||
INFORMATION SCHEMA interface you can query to get information about the
|
||||
database.</P>
|
||||
|
||||
<P>There are also system tables beginning with <I>pg_</I> that describe
|
||||
these too. Use <I>psql -l</I> will list all databases.</P>
|
||||
|
||||
<P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
|
||||
illustrates many of the <SMALL>SELECT</SMALL>s needed to get
|
||||
information from the database system tables.</P>
|
||||
|
||||
<H4><A name="4.4">4.4</A>) How do you remove a column from a
|
||||
table, or change its data type?</H4>
|
||||
|
||||
|
|
@ -935,19 +942,7 @@
|
|||
<P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
|
||||
use very little space.</P>
|
||||
|
||||
<H4><A name="4.7">4.7</A>) How do I find out what tables, indexes,
|
||||
databases, and users are defined?</H4>
|
||||
|
||||
<P><I>psql</I> has a variety of backslash commands to show such
|
||||
information. Use \? to see them. There are also system tables
|
||||
beginning with <I>pg_</I> that describe these too. Also, <I>psql
|
||||
-l</I> will list all databases.</P>
|
||||
|
||||
<P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
|
||||
illustrates many of the <SMALL>SELECT</SMALL>s needed to get
|
||||
information from the database system tables.</P>
|
||||
|
||||
<H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
|
||||
<H4><A name="4.7">4.7</A>) My queries are slow or don't make use of
|
||||
the indexes. Why?</H4>
|
||||
|
||||
<P>Indexes are not automatically used by every query. Indexes are only
|
||||
|
|
@ -998,7 +993,7 @@
|
|||
e.g. [a-e].</LI>
|
||||
<LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
|
||||
<I>~*</I> do not utilise indexes. Instead, use functional
|
||||
indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
|
||||
indexes, which are described in section <a href="#4.11">4.11</a>.</LI>
|
||||
<LI>The default <I>C</I> locale must be used during
|
||||
<i>initdb</i> because it is not possible to know the next-greater
|
||||
character in a non-C locale. You can create a special
|
||||
|
|
@ -1011,12 +1006,12 @@
|
|||
types exactly match the index's column types. This is particularly
|
||||
true of int2, int8, and numeric column indexes.</P>
|
||||
|
||||
<H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
|
||||
<H4><A name="4.8">4.8</A>) How do I see how the query optimizer is
|
||||
evaluating my query?</H4>
|
||||
|
||||
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
|
||||
|
||||
<H4><A name="4.10">4.10</A>) What is an R-tree index?</H4>
|
||||
<H4><A name="4.9">4.9</A>) What is an R-tree index?</H4>
|
||||
|
||||
<P>An R-tree index is used for indexing spatial data. A hash index
|
||||
can't handle range searches. A B-tree index only handles range
|
||||
|
|
@ -1041,7 +1036,7 @@
|
|||
practice, extending R-trees requires a bit of work and we don't
|
||||
currently have any documentation on how to do it.</P>
|
||||
|
||||
<H4><A name="4.11">4.11</A>) What is the Genetic Query
|
||||
<H4><A name="4.10">4.10</A>) What is the Genetic Query
|
||||
Optimizer?</H4>
|
||||
|
||||
<P>The <SMALL>GEQO</SMALL> module speeds query optimization when
|
||||
|
|
@ -1049,7 +1044,7 @@
|
|||
the handling of large join queries through nonexhaustive
|
||||
search.</P>
|
||||
|
||||
<H4><A name="4.12">4.12</A>) How do I perform regular expression
|
||||
<H4><A name="4.11">4.11</A>) How do I perform regular expression
|
||||
searches and case-insensitive regular expression searches? How do I
|
||||
use an index for case-insensitive searches?</H4>
|
||||
|
||||
|
|
@ -1072,13 +1067,13 @@
|
|||
CREATE INDEX tabindex ON tab (lower(col));
|
||||
</PRE>
|
||||
|
||||
<H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
|
||||
<H4><A name="4.12">4.12</A>) In a query, how do I detect if a field
|
||||
is <SMALL>NULL</SMALL>?</H4>
|
||||
|
||||
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
|
||||
NOT NULL</SMALL>.</P>
|
||||
|
||||
<H4><A name="4.14">4.14</A>) What is the difference between the
|
||||
<H4><A name="4.13">4.13</A>) What is the difference between the
|
||||
various character types?</H4>
|
||||
<PRE>
|
||||
Type Internal Name Notes
|
||||
|
|
@ -1110,7 +1105,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
particularly values that include <SMALL>NULL</SMALL> bytes. All the
|
||||
types described here have similar performance characteristics.</P>
|
||||
|
||||
<H4><A name="4.15.1">4.15.1</A>) How do I create a
|
||||
<H4><A name="4.14.1">4.14.1</A>) How do I create a
|
||||
serial/auto-incrementing field?</H4>
|
||||
|
||||
<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
|
||||
|
|
@ -1138,13 +1133,13 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
|
||||
WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
|
||||
|
||||
<H4><A name="4.15.2">4.15.2</A>) How do I get the value of a
|
||||
<H4><A name="4.14.2">4.14.2</A>) How do I get the value of a
|
||||
<SMALL>SERIAL</SMALL> insert?</H4>
|
||||
|
||||
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
|
||||
from the sequence object with the <I>nextval()</I> function
|
||||
<I>before</I> inserting and then insert it explicitly. Using the
|
||||
example table in <A href="#4.15.1">4.15.1</A>, an example in a
|
||||
example table in <A href="#4.14.1">4.14.1</A>, an example in a
|
||||
pseudo-language would look like this:</P>
|
||||
<PRE>
|
||||
new_id = execute("SELECT nextval('person_id_seq')");
|
||||
|
|
@ -1167,7 +1162,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
new_id = execute("SELECT currval('person_id_seq')");
|
||||
</PRE>
|
||||
|
||||
<P>Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
|
||||
<P>Finally, you could use the <A href="#4.15"><SMALL>OID</SMALL></A>
|
||||
returned from the <SMALL>INSERT</SMALL> statement to look up the
|
||||
default value, though this is probably the least portable approach,
|
||||
and the oid value will wrap around when it reaches 4 billion.
|
||||
|
|
@ -1175,13 +1170,13 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
available via <I>$sth->{pg_oid_status}</I> after
|
||||
<I>$sth->execute()</I>.</P>
|
||||
|
||||
<H4><A name="4.15.3">4.15.3</A>) Doesn't <I>currval()</I>
|
||||
<H4><A name="4.14.3">4.14.3</A>) Doesn't <I>currval()</I>
|
||||
lead to a race condition with other users?</H4>
|
||||
|
||||
<P>No. <I>currval()</I> returns the current value assigned by your
|
||||
backend, not by all users.</P>
|
||||
|
||||
<H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
|
||||
<H4><A name="4.14.4">4.14.4</A>) Why aren't my sequence numbers
|
||||
reused on transaction abort? Why are there gaps in the numbering of
|
||||
my sequence/SERIAL column?</H4>
|
||||
|
||||
|
|
@ -1190,7 +1185,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
completes. This causes gaps in numbering from aborted
|
||||
transactions.</P>
|
||||
|
||||
<H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
|
||||
<H4><A name="4.15">4.15</A>) What is an <SMALL>OID</SMALL>? What is
|
||||
a <SMALL>TID</SMALL>?</H4>
|
||||
|
||||
<P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids.
|
||||
|
|
@ -1230,7 +1225,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
are modified or reloaded. They are used by index entries to point
|
||||
to physical rows.</P>
|
||||
|
||||
<H4><A name="4.17">4.17</A>) What is the meaning of some of the
|
||||
<H4><A name="4.16">4.16</A>) What is the meaning of some of the
|
||||
terms used in PostgreSQL?</H4>
|
||||
|
||||
<P>Some of the source code and older documentation use terms that
|
||||
|
|
@ -1259,7 +1254,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
<P>A list of general database terms can be found at: <A href=
|
||||
"http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html">http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html</A></P>
|
||||
|
||||
<H4><A name="4.18">4.18</A>) Why do I get the error <I>"ERROR:
|
||||
<H4><A name="4.17">4.17</A>) Why do I get the error <I>"ERROR:
|
||||
Memory exhausted in AllocSetAlloc()"</I>?</H4>
|
||||
|
||||
<P>You probably have run out of virtual memory on your system,
|
||||
|
|
@ -1278,12 +1273,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
backend is returning too much data, try it before starting the
|
||||
client.
|
||||
|
||||
<H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
|
||||
<H4><A name="4.18">4.18</A>) How do I tell what PostgreSQL version
|
||||
I am running?</H4>
|
||||
|
||||
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
|
||||
|
||||
<H4><A name="4.20">4.20</A>) Why does my large-object operations
|
||||
<H4><A name="4.19">4.19</A>) Why does my large-object operations
|
||||
get <I>"invalid large obj descriptor"</I>?</H4>
|
||||
|
||||
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
|
||||
|
|
@ -1299,7 +1294,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you
|
||||
may need to set <CODE>auto-commit off.</CODE></P>
|
||||
|
||||
<H4><A name="4.21">4.21</A>) How do I create a column that will
|
||||
<H4><A name="4.20">4.20</A>) How do I create a column that will
|
||||
default to the current time?</H4>
|
||||
|
||||
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
|
||||
|
|
@ -1308,7 +1303,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
</CODE>
|
||||
</PRE>
|
||||
|
||||
<H4><A name="4.22">4.22</A>) Why are my subqueries using
|
||||
<H4><A name="4.21">4.21</A>) Why are my subqueries using
|
||||
<CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
|
||||
|
||||
<P>In versions prior to 7.4, subqueries were joined to outer queries
|
||||
|
|
@ -1332,7 +1327,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
sophisticated join techniques as normal queries, and is prefered
|
||||
to using <CODE>EXISTS</CODE>.
|
||||
|
||||
<H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
|
||||
<H4><A name="4.22">4.22</A>) How do I perform an outer join?</H4>
|
||||
|
||||
<P>PostgreSQL supports outer joins using the SQL standard syntax.
|
||||
Here are two examples:</P>
|
||||
|
|
@ -1372,7 +1367,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
ORDER BY col1
|
||||
</PRE>
|
||||
|
||||
<H4><A name="4.24">4.24</A>) How do I perform queries using
|
||||
<H4><A name="4.23">4.23</A>) How do I perform queries using
|
||||
multiple databases?</H4>
|
||||
|
||||
<P>There is no way to query a database other than the current one.
|
||||
|
|
@ -1384,7 +1379,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
connections to different databases and merge the results on the
|
||||
client side.</P>
|
||||
|
||||
<H4><A name="4.25">4.25</A>) How do I return multiple rows or
|
||||
<H4><A name="4.24">4.24</A>) How do I return multiple rows or
|
||||
columns from a function?</H4>
|
||||
|
||||
<P>In 7.3, you can easily return multiple rows or columns from a
|
||||
|
|
@ -1392,7 +1387,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
|
||||
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
|
||||
|
||||
<H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
|
||||
<H4><A name="4.25">4.25</A>) Why can't I reliably create/drop
|
||||
temporary tables in PL/PgSQL functions?</H4>
|
||||
<P>PL/PgSQL caches function contents, and an unfortunate side effect
|
||||
is that if a PL/PgSQL function accesses a temporary table, and that
|
||||
|
|
@ -1402,7 +1397,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
|
|||
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
|
||||
will cause the query to be reparsed every time.</P>
|
||||
|
||||
<H4><A name="4.27">4.27</A>) What encryption options are available?
|
||||
<H4><A name="4.26">4.26</A>) What encryption options are available?
|
||||
</H4>
|
||||
<UL>
|
||||
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
|
||||
|
|
|
|||
Loading…
Reference in New Issue