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Commit2de7ebe

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Remove FAQ about database terms.
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‎doc/FAQ‎

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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Mon Jan 3120:41:21 EST 2005
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Last updated: Mon Jan 3121:31:39 EST 2005
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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@@ -68,19 +68,18 @@
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4.11.3) 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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4.12) What is an OID? What is a TID?
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4.13) What is the meaning of some of the terms used in PostgreSQL?
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4.14) Why do I get the error "ERROR: Memory exhausted in
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4.13) Why do I get the error "ERROR: Memory exhausted in
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AllocSetAlloc()"?
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4.15) How do I tell what PostgreSQL version I am running?
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4.16) Why does my large-object operations get "invalid large obj
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4.14) How do I tell what PostgreSQL version I am running?
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4.15) Why does my large-object operations get "invalid large obj
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descriptor"?
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4.17) How do I create a column that will default to the current time?
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4.18) How do I perform an outer join?
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4.19) How do I perform queries using multiple databases?
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4.20) How do I return multiple rows or columns from a function?
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4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
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4.16) How do I create a column that will default to the current time?
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4.17) How do I perform an outer join?
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4.18) How do I perform queries using multiple databases?
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4.19) How do I return multiple rows or columns from a function?
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4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL
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functions?
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4.22) What encryption options are available?
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4.21) What encryption options are available?
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Extending PostgreSQL
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@@ -394,6 +393,7 @@
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properly. First, by running configure with the --enable-cassert
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option, many assert()s monitor the progress of the backend and halt
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the program when something unexpected occurs.
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The postmaster has a -d option that allows even more detailed
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information to be reported. The -d option takes a number that
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specifies the debug level. Be warned that high debug level values
@@ -517,14 +517,16 @@
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4.4) What is the maximum size for a row, a table, and a database?
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These are the limits:
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Maximum size for a database? unlimited (32 TB databases exist)
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Maximum size for a table? 32 TB
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Maximum size for a row? 1.6TB
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Maximum size for a field? 1 GB
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Maximum number of rows in a table? unlimited
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Maximum number of columns in a table? 250-1600 depending on column types
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Maximum number of indexes on a table? unlimited
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Maximum size for a database? unlimited (32 TB databases exist)
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Maximum size for a table? 32 TB
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Maximum size for a row? 1.6TB
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Maximum size for a field? 1 GB
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Maximum number of rows in a table? unlimited
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Maximum number of columns in a table? 250-1600 depending on column
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types
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Maximum number of indexes on a table? unlimited
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Of course, these are not actually unlimited, but limited to available
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disk space and memory/swap space. Performance may suffer when these
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values get unusually large.
@@ -611,15 +613,15 @@
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* The search string can not start with a character class, e.g.
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[a-e].
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* Case-insensitive searches such as ILIKE and ~* do not utilize
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indexes. Instead, usefunctional indexes, which are described in
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section 4.10.
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indexes. Instead, useexpression indexes, which are described in
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section 4.8.
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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 text_pattern_ops index for such cases that
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work only for LIKE indexing.
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possible to know the next-greatest character in a non-C locale.
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Youcan create a special text_pattern_ops index for such cases
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thatwork only for LIKE indexing.
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In pre-8.0 releases, indexes often can not be used unless the data
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types exactly match the index's column types. Thisis particularly
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types exactly match the index's column types. Thiswas particularly
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true of int2, int8, and numeric column indexes.
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4.7) How do I see how the query optimizer is evaluating my query?
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WHERE lower(col) = 'abc';
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This will not use an standard index. However, if you create a
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functional index, it will be used:
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expresssion index, it will be used:
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CREATE INDEX tabindex ON tab (lower(col));
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4.9) In a query, how do I detect if a field is NULL?
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4.10) 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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VARCHAR(n) varchar size specifies maximum length, no padding
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CHAR(n) bpchar blank padded to the specified fixed length
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TEXT text no specific upper limit on length
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BYTEA bytea variable-length byte array (null-byte safe)
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"char" char one character
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Type Internal Name Notes
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VARCHAR(n) varchar size specifies maximum length, no padding
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CHAR(n) bpchar blank padded to the specified fixed length
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TEXT text no specific upper limit on length
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BYTEA bytea variable-length byte array (null-byte safe)
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"char" char one character
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You will see the internal name when examining system catalogs and in
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some error messages.
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);
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See the create_sequence manual page for more information about
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sequences. You can also use each row's OID field as a unique value.
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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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sequences.
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4.11.2) How do I get the value of a SERIAL insert?
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@@ -716,16 +715,10 @@ BYTEA bytea variable-length byte array (null-byte safe)
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execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
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new_id = execute("SELECT currval('person_id_seq')");
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Finally, you could use the OID returned from the INSERT statement to
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look up the default value, though this is probably the least portable
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approach, and the oid value will wrap around when it reaches 4
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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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4.11.3) Doesn't currval() lead to a race condition with other users?
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No. currval() returns the current value assigned by yourbackend, not
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by allusers.
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No. currval() returns the current value assigned by yoursession, not
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by allsessions.
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4.11.4) 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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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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4.13) 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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* table, relation, class
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* row, record, tuple
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* column, field, attribute
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* retrieve, select
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* replace, update
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* append, insert
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* OID, serial value
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* portal, cursor
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* range variable, table name, table alias
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A list of general database terms can be found at:
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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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4.14) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
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4.13) 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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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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4.15) How do I tell what PostgreSQL version I am running?
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4.14) How do I tell what PostgreSQL version I am running?
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From psql, type SELECT version();
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4.16) Why does my large-object operations get "invalid large obj
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4.15) 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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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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4.17) How do I create a column that will default to the current time?
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4.16) 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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4.18) How do I perform an outer join?
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4.17) 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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WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
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ORDER BY col1
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4.19) How do I perform queries using multiple databases?
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4.18) 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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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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4.20) How do I return multiple rows or columns from a function?
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4.19) 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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4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
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4.20) 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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table access in PL/PgSQL. This will cause the query to be reparsed
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every time.
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4.22) What encryption options are available?
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4.21) 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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