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Commitbfc5362

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Remove GEQO and Rtree FAQ items.
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‎doc/FAQ‎

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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Sat Jan 29 23:06:02 EST 2005
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Last updated: Sat Jan 29 23:15:42 EST 2005
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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@@ -58,33 +58,31 @@
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typical text file?
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4.6) Why are my queries slow? Why don't they use my indexes?
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4.7) How do I see how the query optimizer is evaluating my query?
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4.8) What is an R-tree index?
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4.9) What is the Genetic Query Optimizer?
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4.10) 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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4.11) In a query, how do I detect if a field is NULL?
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4.12) What is the difference between the various character types?
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4.13.0) How do I create a serial/auto-incrementing field?
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4.13.1) How do I get the value of a SERIAL insert?
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4.13.2) Doesn't currval() lead to a race condition with other users?
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4.13.3) Why aren't my sequence numbers reused on transaction abort?
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4.8) How do I perform regular expression searches and case-insensitive
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regular expression searches? How do I use an index for
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case-insensitive searches?
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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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4.11.0) How do I create a serial/auto-incrementing field?
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4.11.1) How do I get the value of a SERIAL insert?
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4.11.2) Doesn't currval() lead to a race condition with other users?
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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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4.14) What is an OID? What is a TID?
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4.15) What is the meaning of some of the terms used in PostgreSQL?
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4.16) Why do I get the error "ERROR: Memory exhausted in
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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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AllocSetAlloc()"?
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4.17) How do I tell what PostgreSQL version I am running?
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4.18) Why does my large-object operations get "invalid large obj
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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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descriptor"?
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4.19) How do I create a column that will default to the current time?
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4.20) Why are my subqueries using IN so slow?
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4.21) How do I perform an outer join?
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4.22) How do I perform queries using multiple databases?
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4.23) How do I return multiple rows or columns from a function?
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4.24) Why can't I reliably create/drop temporary tables in PL/PgSQL
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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) Why are my subqueries using IN so slow?
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4.19) How do I perform an outer join?
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4.20) How do I perform queries using multiple databases?
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4.21) How do I return multiple rows or columns from a function?
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4.22) Why can't I reliably create/drop temporary tables in PL/PgSQL
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functions?
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4.25) What encryption options are available?
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4.23) What encryption options are available?
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Extending PostgreSQL
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@@ -742,36 +740,7 @@ LIKE
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See the EXPLAIN manual page.
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4.8) 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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single dimension. R-trees can handle multi-dimensional data. For
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example, if an R-tree index can be built on an attribute of type
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point, the system can more efficiently answer queries such as "select
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all points within a bounding rectangle."
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The canonical paper that describes the original R-tree design is:
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Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
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Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt of
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Data, 45-57.
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You can also find this paper in Stonebraker's "Readings in Database
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Systems".
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Built-in R-trees can handle polygons and boxes. In theory, R-trees can
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be extended to handle higher number of dimensions. In practice,
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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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4.9) 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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4.10) How do I perform regular expression searches and case-insensitive
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4.8) 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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@@ -788,11 +757,11 @@ 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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4.11) In a query, how do I detect if a field is NULL?
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4.9) 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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4.12) What is the difference between the various character types?
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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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--------------------------------------------------
@@ -820,7 +789,7 @@ 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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4.13.1) How do I create a serial/auto-incrementing field?
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4.11.1) 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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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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4.13.2) How do I get the value of a SERIAL insert?
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4.11.2) 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 4.13.1, an example in a
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explicitly. Using the example table in 4.11.1, 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')");
@@ -867,19 +836,19 @@ 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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4.13.3) Doesn't currval() lead to a race condition with other users?
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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 your backend, not
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by all users.
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4.13.4) Why aren't my sequence numbers reused on transaction abort? Why are
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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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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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4.14) What is an OID? What is a TID?
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4.12) What is an OID? What is a TID?
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Every row that is created in PostgreSQL gets a unique OID unless
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created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
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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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4.15) What is the meaning of some of the terms used in PostgreSQL?
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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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http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
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/glossary.html
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4.16) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
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4.14) 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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4.17) How do I tell what PostgreSQL version I am running?
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4.15) How do I tell what PostgreSQL version I am running?
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From psql, type SELECT version();
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4.18) Why does my large-object operations get "invalid large obj
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4.16) 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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4.19) How do I create a column that will default to the current time?
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4.17) 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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4.20) Why are my subqueries using IN so slow?
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4.18) 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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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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4.21) How do I perform an outer join?
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4.19) 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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4.22) How do I perform queries using multiple databases?
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4.20) 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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4.23) How do I return multiple rows or columns from a function?
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4.21) 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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4.24) Why can't I reliably create/drop temporary tables in PL/PgSQL
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4.22) 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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4.25) What encryption options are available?
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4.23) 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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