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Remove FAQ item about binary cursors.

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Frequently Asked Questions (FAQ) for PostgreSQL Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sat Jan 29 22:59:12 EST 2005 Last updated: Sat Jan 29 23:02:37 EST 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
...@@ -49,43 +49,42 @@ ...@@ -49,43 +49,42 @@
Operational Questions Operational Questions
4.1) What is the difference between binary cursors and normal cursors? 4.1) How do I SELECT only the first few rows of a query? A random row?
4.2) How do I SELECT only the first few rows of a query? A random row? 4.2) How do I find out what tables, indexes, databases, and users are
4.3) How do I find out what tables, indexes, databases, and users are
defined? How do I see the queries used by psql to display them? defined? How do I see the queries used by psql to display them?
4.4) How do you remove a column from a table, or change its data type? 4.3) How do you remove a column from a table, or change its data type?
4.5) What is the maximum size for a row, a table, and a database? 4.4) What is the maximum size for a row, a table, and a database?
4.6) How much database disk space is required to store data from a 4.5) How much database disk space is required to store data from a
typical text file? typical text file?
4.7) My queries are slow or don't make use of the indexes. Why? 4.6) My queries are slow or don't make use of the indexes. Why?
4.8) How do I see how the query optimizer is evaluating my query? 4.7) How do I see how the query optimizer is evaluating my query?
4.9) What is an R-tree index? 4.8) What is an R-tree index?
4.10) What is the Genetic Query Optimizer? 4.9) What is the Genetic Query Optimizer?
4.11) How do I perform regular expression searches and 4.10) How do I perform regular expression searches and
case-insensitive regular expression searches? How do I use an index case-insensitive regular expression searches? How do I use an index
for case-insensitive searches? for case-insensitive searches?
4.12) In a query, how do I detect if a field is NULL? 4.11) In a query, how do I detect if a field is NULL?
4.13) What is the difference between the various character types? 4.12) What is the difference between the various character types?
4.14.0) How do I create a serial/auto-incrementing field? 4.13.0) How do I create a serial/auto-incrementing field?
4.14.1) How do I get the value of a SERIAL insert? 4.13.1) How do I get the value of a SERIAL insert?
4.14.2) Doesn't currval() lead to a race condition with other users? 4.13.2) Doesn't currval() lead to a race condition with other users?
4.14.3) Why aren't my sequence numbers reused on transaction abort? 4.13.3) Why aren't my sequence numbers reused on transaction abort?
Why are there gaps in the numbering of my sequence/SERIAL column? Why are there gaps in the numbering of my sequence/SERIAL column?
4.15) What is an OID? What is a TID? 4.14) What is an OID? What is a TID?
4.16) What is the meaning of some of the terms used in PostgreSQL? 4.15) What is the meaning of some of the terms used in PostgreSQL?
4.17) Why do I get the error "ERROR: Memory exhausted in 4.16) Why do I get the error "ERROR: Memory exhausted in
AllocSetAlloc()"? AllocSetAlloc()"?
4.18) How do I tell what PostgreSQL version I am running? 4.17) How do I tell what PostgreSQL version I am running?
4.19) Why does my large-object operations get "invalid large obj 4.18) Why does my large-object operations get "invalid large obj
descriptor"? descriptor"?
4.20) How do I create a column that will default to the current time? 4.19) How do I create a column that will default to the current time?
4.21) Why are my subqueries using IN so slow? 4.20) Why are my subqueries using IN so slow?
4.22) How do I perform an outer join? 4.21) How do I perform an outer join?
4.23) How do I perform queries using multiple databases? 4.22) How do I perform queries using multiple databases?
4.24) How do I return multiple rows or columns from a function? 4.23) How do I return multiple rows or columns from a function?
4.25) Why can't I reliably create/drop temporary tables in PL/PgSQL 4.24) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions? functions?
4.26) What encryption options are available? 4.25) What encryption options are available?
Extending PostgreSQL Extending PostgreSQL
...@@ -567,11 +566,7 @@ log_* ...@@ -567,11 +566,7 @@ log_*
Operational Questions Operational Questions
4.1) What is the difference between binary cursors and normal cursors? 4.1) How do I SELECT only the first few rows of a query? A random row?
See the DECLARE manual page for a description.
4.2) How do I SELECT only the first few rows of a query? A random row?
See the FETCH manual page, or use SELECT ... LIMIT.... See the FETCH manual page, or use SELECT ... LIMIT....
...@@ -587,7 +582,7 @@ log_* ...@@ -587,7 +582,7 @@ log_*
ORDER BY random() ORDER BY random()
LIMIT 1; LIMIT 1;
4.3) How do I find out what tables, indexes, databases, and users are 4.2) How do I find out what tables, indexes, databases, and users are
defined? How do I see the queries used by psql to display them? defined? How do I see the queries used by psql to display them?
Use the \dt command to see tables in psql. For a complete list of Use the \dt command to see tables in psql. For a complete list of
...@@ -606,7 +601,7 @@ log_* ...@@ -606,7 +601,7 @@ log_*
many of the SELECTs needed to get information from the database system many of the SELECTs needed to get information from the database system
tables. tables.
4.4) How do you remove a column from a table, or change its data type? 4.3) How do you remove a column from a table, or change its data type?
DROP COLUMN functionality was added in release 7.3 with ALTER TABLE DROP COLUMN functionality was added in release 7.3 with ALTER TABLE
DROP COLUMN. In earlier versions, you can do this: DROP COLUMN. In earlier versions, you can do this:
...@@ -632,7 +627,7 @@ log_* ...@@ -632,7 +627,7 @@ log_*
You might then want to do VACUUM FULL tab to reclaim the disk space You might then want to do VACUUM FULL tab to reclaim the disk space
used by the expired rows. used by the expired rows.
4.5) What is the maximum size for a row, a table, and a database? 4.4) What is the maximum size for a row, a table, and a database?
These are the limits: These are the limits:
Maximum size for a database? unlimited (32 TB databases exist) Maximum size for a database? unlimited (32 TB databases exist)
...@@ -654,7 +649,7 @@ log_* ...@@ -654,7 +649,7 @@ log_*
The maximum table size and maximum number of columns can be quadrupled The maximum table size and maximum number of columns can be quadrupled
by increasing the default block size to 32k. by increasing the default block size to 32k.
4.6) How much database disk space is required to store data from a typical 4.5) How much database disk space is required to store data from a typical
text file? text file?
A PostgreSQL database may require up to five times the disk space to A PostgreSQL database may require up to five times the disk space to
...@@ -688,7 +683,7 @@ log_* ...@@ -688,7 +683,7 @@ log_*
NULLs are stored as bitmaps, so they use very little space. NULLs are stored as bitmaps, so they use very little space.
4.7) My queries are slow or don't make use of the indexes. Why? 4.6) My queries are slow or don't make use of the indexes. Why?
Indexes are not automatically used by every query. Indexes are only Indexes are not automatically used by every query. Indexes are only
used if the table is larger than a minimum size, and the query selects used if the table is larger than a minimum size, and the query selects
...@@ -730,7 +725,7 @@ log_* ...@@ -730,7 +725,7 @@ log_*
[a-e]. [a-e].
* Case-insensitive searches such as ILIKE and ~* do not utilize * Case-insensitive searches such as ILIKE and ~* do not utilize
indexes. Instead, use functional indexes, which are described in indexes. Instead, use functional indexes, which are described in
section 4.11. section 4.10.
* The default C locale must be used during initdb because it is not * The default C locale must be used during initdb because it is not
possible to know the next-greater character in a non-C locale. You possible to know the next-greater character in a non-C locale. You
can create a special can create a special
...@@ -743,11 +738,11 @@ LIKE ...@@ -743,11 +738,11 @@ LIKE
types exactly match the index's column types. This is particularly types exactly match the index's column types. This is particularly
true of int2, int8, and numeric column indexes. true of int2, int8, and numeric column indexes.
4.8) How do I see how the query optimizer is evaluating my query? 4.7) How do I see how the query optimizer is evaluating my query?
See the EXPLAIN manual page. See the EXPLAIN manual page.
4.9) What is an R-tree index? 4.8) What is an R-tree index?
An R-tree index is used for indexing spatial data. A hash index can't 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 searches in a handle range searches. A B-tree index only handles range searches in a
...@@ -770,13 +765,13 @@ LIKE ...@@ -770,13 +765,13 @@ LIKE
extending R-trees requires a bit of work and we don't currently have extending R-trees requires a bit of work and we don't currently have
any documentation on how to do it. any documentation on how to do it.
4.10) What is the Genetic Query Optimizer? 4.9) What is the Genetic Query Optimizer?
The GEQO module speeds query optimization when joining many tables by The GEQO module speeds query optimization when joining many tables by
means of a Genetic Algorithm (GA). It allows the handling of large means of a Genetic Algorithm (GA). It allows the handling of large
join queries through nonexhaustive search. join queries through nonexhaustive search.
4.11) How do I perform regular expression searches and case-insensitive 4.10) How do I perform regular expression searches and case-insensitive
regular expression searches? How do I use an index for case-insensitive regular expression searches? How do I use an index for case-insensitive
searches? searches?
...@@ -793,11 +788,11 @@ LIKE ...@@ -793,11 +788,11 @@ LIKE
functional index, it will be used: functional index, it will be used:
CREATE INDEX tabindex ON tab (lower(col)); CREATE INDEX tabindex ON tab (lower(col));
4.12) In a query, how do I detect if a field is NULL? 4.11) In a query, how do I detect if a field is NULL?
You test the column with IS NULL and IS NOT NULL. You test the column with IS NULL and IS NOT NULL.
4.13) What is the difference between the various character types? 4.12) What is the difference between the various character types?
Type Internal Name Notes Type Internal Name Notes
-------------------------------------------------- --------------------------------------------------
...@@ -825,7 +820,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -825,7 +820,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
particularly values that include NULL bytes. All the types described particularly values that include NULL bytes. All the types described
here have similar performance characteristics. here have similar performance characteristics.
4.14.1) How do I create a serial/auto-incrementing field? 4.13.1) How do I create a serial/auto-incrementing field?
PostgreSQL supports a SERIAL data type. It auto-creates a sequence. PostgreSQL supports a SERIAL data type. It auto-creates a sequence.
For example, this: For example, this:
...@@ -846,11 +841,11 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -846,11 +841,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
However, if you need to dump and reload the database, you need to use However, if you need to dump and reload the database, you need to use
pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs. pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
4.14.2) How do I get the value of a SERIAL insert? 4.13.2) How do I get the value of a SERIAL insert?
One approach is to retrieve the next SERIAL value from the sequence One approach is to retrieve the next SERIAL value from the sequence
object with the nextval() function before inserting and then insert it object with the nextval() function before inserting and then insert it
explicitly. Using the example table in 4.14.1, an example in a explicitly. Using the example table in 4.13.1, an example in a
pseudo-language would look like this: pseudo-language would look like this:
new_id = execute("SELECT nextval('person_id_seq')"); new_id = execute("SELECT nextval('person_id_seq')");
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')"); execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
...@@ -872,19 +867,19 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -872,19 +867,19 @@ BYTEA bytea variable-length byte array (null-byte safe)
billion. In Perl, using DBI with the DBD::Pg module, the oid value is billion. In Perl, using DBI with the DBD::Pg module, the oid value is
made available via $sth->{pg_oid_status} after $sth->execute(). made available via $sth->{pg_oid_status} after $sth->execute().
4.14.3) Doesn't currval() lead to a race condition with other users? 4.13.3) Doesn't currval() lead to a race condition with other users?
No. currval() returns the current value assigned by your backend, not No. currval() returns the current value assigned by your backend, not
by all users. by all users.
4.14.4) Why aren't my sequence numbers reused on transaction abort? Why are 4.13.4) Why aren't my sequence numbers reused on transaction abort? Why are
there gaps in the numbering of my sequence/SERIAL column? there gaps in the numbering of my sequence/SERIAL column?
To improve concurrency, sequence values are given out to running To improve concurrency, sequence values are given out to running
transactions as needed and are not locked until the transaction transactions as needed and are not locked until the transaction
completes. This causes gaps in numbering from aborted transactions. completes. This causes gaps in numbering from aborted transactions.
4.15) What is an OID? What is a TID? 4.14) What is an OID? What is a TID?
Every row that is created in PostgreSQL gets a unique OID unless Every row that is created in PostgreSQL gets a unique OID unless
created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
...@@ -901,7 +896,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -901,7 +896,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
values. TIDs change after rows are modified or reloaded. They are used values. TIDs change after rows are modified or reloaded. They are used
by index entries to point to physical rows. by index entries to point to physical rows.
4.16) What is the meaning of some of the terms used in PostgreSQL? 4.15) What is the meaning of some of the terms used in PostgreSQL?
Some of the source code and older documentation use terms that have Some of the source code and older documentation use terms that have
more common usage. Here are some: more common usage. Here are some:
...@@ -919,7 +914,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -919,7 +914,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
/glossary.html /glossary.html
4.17) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"? 4.16) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
You probably have run out of virtual memory on your system, or your You probably have run out of virtual memory on your system, or your
kernel has a low limit for certain resources. Try this before starting kernel has a low limit for certain resources. Try this before starting
...@@ -934,11 +929,11 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -934,11 +929,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
problem with the SQL client because the backend is returning too much problem with the SQL client because the backend is returning too much
data, try it before starting the client. data, try it before starting the client.
4.18) How do I tell what PostgreSQL version I am running? 4.17) How do I tell what PostgreSQL version I am running?
From psql, type SELECT version(); From psql, type SELECT version();
4.19) Why does my large-object operations get "invalid large obj 4.18) Why does my large-object operations get "invalid large obj
descriptor"? descriptor"?
You need to put BEGIN WORK and COMMIT around any use of a large object You need to put BEGIN WORK and COMMIT around any use of a large object
...@@ -953,12 +948,12 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -953,12 +948,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
If you are using a client interface like ODBC you may need to set If you are using a client interface like ODBC you may need to set
auto-commit off. auto-commit off.
4.20) How do I create a column that will default to the current time? 4.19) How do I create a column that will default to the current time?
Use CURRENT_TIMESTAMP: Use CURRENT_TIMESTAMP:
CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP ); CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
4.21) Why are my subqueries using IN so slow? 4.20) Why are my subqueries using IN so slow?
In versions prior to 7.4, subqueries were joined to outer queries by In versions prior to 7.4, subqueries were joined to outer queries by
sequentially scanning the result of the subquery for each row of the sequentially scanning the result of the subquery for each row of the
...@@ -979,7 +974,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP ); ...@@ -979,7 +974,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
In version 7.4 and later, IN actually uses the same sophisticated join In version 7.4 and later, IN actually uses the same sophisticated join
techniques as normal queries, and is prefered to using EXISTS. techniques as normal queries, and is prefered to using EXISTS.
4.22) How do I perform an outer join? 4.21) How do I perform an outer join?
PostgreSQL supports outer joins using the SQL standard syntax. Here PostgreSQL supports outer joins using the SQL standard syntax. Here
are two examples: are two examples:
...@@ -1009,7 +1004,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP ); ...@@ -1009,7 +1004,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2) WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
ORDER BY col1 ORDER BY col1
4.23) How do I perform queries using multiple databases? 4.22) How do I perform queries using multiple databases?
There is no way to query a database other than the current one. There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is Because PostgreSQL loads database-specific system catalogs, it is
...@@ -1019,12 +1014,12 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP ); ...@@ -1019,12 +1014,12 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
course, a client can make simultaneous connections to different course, a client can make simultaneous connections to different
databases and merge the results on the client side. databases and merge the results on the client side.
4.24) How do I return multiple rows or columns from a function? 4.23) How do I return multiple rows or columns from a function?
In 7.3, you can easily return multiple rows or columns from a In 7.3, you can easily return multiple rows or columns from a
function, http://techdocs.postgresql.org/guides/SetReturningFunctions. function, http://techdocs.postgresql.org/guides/SetReturningFunctions.
4.25) Why can't I reliably create/drop temporary tables in PL/PgSQL 4.24) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions? functions?
PL/PgSQL caches function contents, and an unfortunate side effect is PL/PgSQL caches function contents, and an unfortunate side effect is
...@@ -1035,7 +1030,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP ); ...@@ -1035,7 +1030,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
table access in PL/PgSQL. This will cause the query to be reparsed table access in PL/PgSQL. This will cause the query to be reparsed
every time. every time.
4.26) What encryption options are available? 4.25) What encryption options are available?
* contrib/pgcrypto contains many encryption functions for use in SQL * contrib/pgcrypto contains many encryption functions for use in SQL
queries. queries.
......
doc/src/FAQ/FAQ.html
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...@@ -10,7 +10,7 @@ ...@@ -10,7 +10,7 @@
alink="#0000ff"> alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1> <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Sat Jan 29 22:59:12 EST 2005</P> <P>Last updated: Sat Jan 29 23:02:37 EST 2005</P>
<P>Current maintainer: Bruce Momjian (<A href= <P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR> "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
...@@ -71,63 +71,61 @@ ...@@ -71,63 +71,61 @@
<H2 align="center">Operational Questions</H2> <H2 align="center">Operational Questions</H2>
<A href="#4.1">4.1</A>) What is the difference between binary <A href="#4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
cursors and normal cursors?<BR>
<A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?<BR> first few rows of a query? A random row?<BR>
<A href="#4.3">4.3</A>) How do I find out what tables, indexes, <A href="#4.2">4.2</A>) How do I find out what tables, indexes,
databases, and users are defined? How do I see the queries used databases, and users are defined? How do I see the queries used
by <I>psql</I> to display them?<BR> by <I>psql</I> to display them?<BR>
<A href="#4.4">4.4</A>) How do you remove a column from a <A href="#4.3">4.3</A>) How do you remove a column from a
table, or change its data type?<BR> table, or change its data type?<BR>
<A href="#4.5">4.5</A>) What is the maximum size for a row, a <A href="#4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?<BR> table, and a database?<BR>
<A href="#4.6">4.6</A>) How much database disk space is required <A href="#4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?<BR> to store data from a typical text file?<BR>
<A href="#4.7">4.7</A>) My queries are slow or don't make use of <A href="#4.6">4.6</A>) My queries are slow or don't make use of
the indexes. Why?<BR> the indexes. Why?<BR>
<A href="#4.8">4.8</A>) How do I see how the query optimizer is <A href="#4.7">4.7</A>) How do I see how the query optimizer is
evaluating my query?<BR> evaluating my query?<BR>
<A href="#4.9">4.9</A>) What is an R-tree index?<BR> <A href="#4.8">4.8</A>) What is an R-tree index?<BR>
<A href="#4.10">4.10</A>) What is the Genetic Query Optimizer?<BR> <A href="#4.9">4.9</A>) What is the Genetic Query Optimizer?<BR>
<A href="#4.11">4.11</A>) How do I perform regular expression <A href="#4.10">4.10</A>) How do I perform regular expression
searches and case-insensitive regular expression searches? How do I searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?<BR> use an index for case-insensitive searches?<BR>
<A href="#4.12">4.12</A>) In a query, how do I detect if a field <A href="#4.11">4.11</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>?<BR> is <SMALL>NULL</SMALL>?<BR>
<A href="#4.13">4.13</A>) What is the difference between the <A href="#4.12">4.12</A>) What is the difference between the
various character types?<BR> various character types?<BR>
<A href="#4.14.0">4.14.0</A>) How do I create a <A href="#4.13.0">4.13.0</A>) How do I create a
serial/auto-incrementing field?<BR> serial/auto-incrementing field?<BR>
<A href="#4.14.1">4.14.1</A>) How do I get the value of a <A href="#4.13.1">4.13.1</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?<BR> <SMALL>SERIAL</SMALL> insert?<BR>
<A href="#4.14.2">4.14.2</A>) Doesn't <I>currval()</I> <A href="#4.13.2">4.13.2</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?<BR> lead to a race condition with other users?<BR>
<A href="#4.14.3">4.14.3</A>) Why aren't my sequence numbers <A href="#4.13.3">4.13.3</A>) Why aren't my sequence numbers
reused on transaction abort? Why are there gaps in the numbering of reused on transaction abort? Why are there gaps in the numbering of
my sequence/SERIAL column?<BR> my sequence/SERIAL column?<BR>
<A href="#4.15">4.15</A>) What is an <SMALL>OID</SMALL>? What is a <A href="#4.14">4.14</A>) What is an <SMALL>OID</SMALL>? What is a
<SMALL>TID</SMALL>?<BR> <SMALL>TID</SMALL>?<BR>
<A href="#4.16">4.16</A>) What is the meaning of some of the terms <A href="#4.15">4.15</A>) What is the meaning of some of the terms
used in PostgreSQL?<BR> used in PostgreSQL?<BR>
<A href="#4.17">4.17</A>) Why do I get the error <I>"ERROR: Memory <A href="#4.16">4.16</A>) Why do I get the error <I>"ERROR: Memory
exhausted in AllocSetAlloc()"</I>?<BR> exhausted in AllocSetAlloc()"</I>?<BR>
<A href="#4.18">4.18</A>) How do I tell what PostgreSQL version I <A href="#4.17">4.17</A>) How do I tell what PostgreSQL version I
am running?<BR> am running?<BR>
<A href="#4.19">4.19</A>) Why does my large-object operations get <A href="#4.18">4.18</A>) Why does my large-object operations get
<I>"invalid large obj descriptor"</I>?<BR> <I>"invalid large obj descriptor"</I>?<BR>
<A href="#4.20">4.20</A>) How do I create a column that will <A href="#4.19">4.19</A>) How do I create a column that will
default to the current time?<BR> default to the current time?<BR>
<A href="#4.21">4.21</A>) Why are my subqueries using <A href="#4.20">4.20</A>) Why are my subqueries using
<CODE><SMALL>IN</SMALL></CODE> so slow?<BR> <CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
<A href="#4.22">4.22</A>) How do I perform an outer join?<BR> <A href="#4.21">4.21</A>) How do I perform an outer join?<BR>
<A href="#4.23">4.23</A>) How do I perform queries using multiple <A href="#4.22">4.22</A>) How do I perform queries using multiple
databases?<BR> databases?<BR>
<A href="#4.24">4.24</A>) How do I return multiple rows or columns <A href="#4.23">4.23</A>) How do I return multiple rows or columns
from a function?<BR> from a function?<BR>
<A href="#4.25">4.25</A>) Why can't I reliably create/drop <A href="#4.24">4.24</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?<BR> temporary tables in PL/PgSQL functions?<BR>
<A href="#4.26">4.26</A>) What encryption options are available?<BR> <A href="#4.25">4.25</A>) What encryption options are available?<BR>
<H2 align="center">Extending PostgreSQL</H2> <H2 align="center">Extending PostgreSQL</H2>
...@@ -712,13 +710,7 @@ ...@@ -712,13 +710,7 @@
<H2 align="center">Operational Questions</H2> <H2 align="center">Operational Questions</H2>
<H4><A name="4.1">4.1</A>) What is the difference between binary <H4><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
cursors and normal cursors?</H4>
<P>See the <SMALL>DECLARE</SMALL> manual page for a
description.</P>
<H4><A name="4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?</H4> first few rows of a query? A random row?</H4>
<P>See the <SMALL>FETCH</SMALL> manual page, or use <P>See the <SMALL>FETCH</SMALL> manual page, or use
...@@ -739,7 +731,7 @@ ...@@ -739,7 +731,7 @@
LIMIT 1; LIMIT 1;
</PRE> </PRE>
<H4><A name="4.3">4.3</A>) How do I find out what tables, indexes, <H4><A name="4.2">4.2</A>) How do I find out what tables, indexes,
databases, and users are defined? How do I see the queries used databases, and users are defined? How do I see the queries used
by <I>psql</I> to display them?</H4> by <I>psql</I> to display them?</H4>
...@@ -760,7 +752,7 @@ ...@@ -760,7 +752,7 @@
illustrates many of the <SMALL>SELECT</SMALL>s needed to get illustrates many of the <SMALL>SELECT</SMALL>s needed to get
information from the database system tables.</P> information from the database system tables.</P>
<H4><A name="4.4">4.4</A>) How do you remove a column from a <H4><A name="4.3">4.3</A>) How do you remove a column from a
table, or change its data type?</H4> table, or change its data type?</H4>
<P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3 <P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3
...@@ -791,7 +783,7 @@ ...@@ -791,7 +783,7 @@
<P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the <P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
disk space used by the expired rows.</P> disk space used by the expired rows.</P>
<H4><A name="4.5">4.5</A>) What is the maximum size for a row, a <H4><A name="4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?</H4> table, and a database?</H4>
<P>These are the limits:</P> <P>These are the limits:</P>
...@@ -817,7 +809,7 @@ ...@@ -817,7 +809,7 @@
<P>The maximum table size and maximum number of columns can be <P>The maximum table size and maximum number of columns can be
quadrupled by increasing the default block size to 32k.</P> quadrupled by increasing the default block size to 32k.</P>
<H4><A name="4.6">4.6</A>) How much database disk space is required <H4><A name="4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?</H4> to store data from a typical text file?</H4>
<P>A PostgreSQL database may require up to five times the disk <P>A PostgreSQL database may require up to five times the disk
...@@ -854,7 +846,7 @@ ...@@ -854,7 +846,7 @@
<P><SMALL>NULL</SMALL>s are stored as bitmaps, so they <P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
use very little space.</P> use very little space.</P>
<H4><A name="4.7">4.7</A>) My queries are slow or don't make use of <H4><A name="4.6">4.6</A>) My queries are slow or don't make use of
the indexes. Why?</H4> the indexes. Why?</H4>
<P>Indexes are not automatically used by every query. Indexes are only <P>Indexes are not automatically used by every query. Indexes are only
...@@ -905,7 +897,7 @@ ...@@ -905,7 +897,7 @@
e.g. [a-e].</LI> e.g. [a-e].</LI>
<LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
<I>~*</I> do not utilize indexes. Instead, use functional <I>~*</I> do not utilize indexes. Instead, use functional
indexes, which are described in section <a href="#4.11">4.11</a>.</LI> indexes, which are described in section <a href="#4.10">4.10</a>.</LI>
<LI>The default <I>C</I> locale must be used during <LI>The default <I>C</I> locale must be used during
<i>initdb</i> because it is not possible to know the next-greater <i>initdb</i> because it is not possible to know the next-greater
character in a non-C locale. You can create a special character in a non-C locale. You can create a special
...@@ -918,12 +910,12 @@ ...@@ -918,12 +910,12 @@
types exactly match the index's column types. This is particularly types exactly match the index's column types. This is particularly
true of int2, int8, and numeric column indexes.</P> true of int2, int8, and numeric column indexes.</P>
<H4><A name="4.8">4.8</A>) How do I see how the query optimizer is <H4><A name="4.7">4.7</A>) How do I see how the query optimizer is
evaluating my query?</H4> evaluating my query?</H4>
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P> <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
<H4><A name="4.9">4.9</A>) What is an R-tree index?</H4> <H4><A name="4.8">4.8</A>) What is an R-tree index?</H4>
<P>An R-tree index is used for indexing spatial data. A hash index <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 can't handle range searches. A B-tree index only handles range
...@@ -948,7 +940,7 @@ ...@@ -948,7 +940,7 @@
practice, extending R-trees requires a bit of work and we don't practice, extending R-trees requires a bit of work and we don't
currently have any documentation on how to do it.</P> currently have any documentation on how to do it.</P>
<H4><A name="4.10">4.10</A>) What is the Genetic Query <H4><A name="4.9">4.9</A>) What is the Genetic Query
Optimizer?</H4> Optimizer?</H4>
<P>The <SMALL>GEQO</SMALL> module speeds query optimization when <P>The <SMALL>GEQO</SMALL> module speeds query optimization when
...@@ -956,7 +948,7 @@ ...@@ -956,7 +948,7 @@
the handling of large join queries through nonexhaustive the handling of large join queries through nonexhaustive
search.</P> search.</P>
<H4><A name="4.11">4.11</A>) How do I perform regular expression <H4><A name="4.10">4.10</A>) How do I perform regular expression
searches and case-insensitive regular expression searches? How do I searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?</H4> use an index for case-insensitive searches?</H4>
...@@ -979,13 +971,13 @@ ...@@ -979,13 +971,13 @@
CREATE INDEX tabindex ON tab (lower(col)); CREATE INDEX tabindex ON tab (lower(col));
</PRE> </PRE>
<H4><A name="4.12">4.12</A>) In a query, how do I detect if a field <H4><A name="4.11">4.11</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>?</H4> is <SMALL>NULL</SMALL>?</H4>
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
NOT NULL</SMALL>.</P> NOT NULL</SMALL>.</P>
<H4><A name="4.13">4.13</A>) What is the difference between the <H4><A name="4.12">4.12</A>) What is the difference between the
various character types?</H4> various character types?</H4>
<PRE> <PRE>
Type Internal Name Notes Type Internal Name Notes
...@@ -1017,7 +1009,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1017,7 +1009,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
particularly values that include <SMALL>NULL</SMALL> bytes. All the particularly values that include <SMALL>NULL</SMALL> bytes. All the
types described here have similar performance characteristics.</P> types described here have similar performance characteristics.</P>
<H4><A name="4.14.1">4.14.1</A>) How do I create a <H4><A name="4.13.1">4.13.1</A>) How do I create a
serial/auto-incrementing field?</H4> serial/auto-incrementing field?</H4>
<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
...@@ -1045,13 +1037,13 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1045,13 +1037,13 @@ 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 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. WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
<H4><A name="4.14.2">4.14.2</A>) How do I get the value of a <H4><A name="4.13.2">4.13.2</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H4> <SMALL>SERIAL</SMALL> insert?</H4>
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
from the sequence object with the <I>nextval()</I> function from the sequence object with the <I>nextval()</I> function
<I>before</I> inserting and then insert it explicitly. Using the <I>before</I> inserting and then insert it explicitly. Using the
example table in <A href="#4.14.1">4.14.1</A>, an example in a example table in <A href="#4.13.1">4.13.1</A>, an example in a
pseudo-language would look like this:</P> pseudo-language would look like this:</P>
<PRE> <PRE>
new_id = execute("SELECT nextval('person_id_seq')"); new_id = execute("SELECT nextval('person_id_seq')");
...@@ -1074,7 +1066,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1074,7 +1066,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
new_id = execute("SELECT currval('person_id_seq')"); new_id = execute("SELECT currval('person_id_seq')");
</PRE> </PRE>
<P>Finally, you could use the <A href="#4.15"><SMALL>OID</SMALL></A> <P>Finally, you could use the <A href="#4.14"><SMALL>OID</SMALL></A>
returned from the <SMALL>INSERT</SMALL> statement to look up the returned from the <SMALL>INSERT</SMALL> statement to look up the
default value, though this is probably the least portable approach, default value, though this is probably the least portable approach,
and the oid value will wrap around when it reaches 4 billion. and the oid value will wrap around when it reaches 4 billion.
...@@ -1082,13 +1074,13 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1082,13 +1074,13 @@ BYTEA bytea variable-length byte array (null-byte safe)
available via <I>$sth-&gt;{pg_oid_status}</I> after available via <I>$sth-&gt;{pg_oid_status}</I> after
<I>$sth-&gt;execute()</I>.</P> <I>$sth-&gt;execute()</I>.</P>
<H4><A name="4.14.3">4.14.3</A>) Doesn't <I>currval()</I> <H4><A name="4.13.3">4.13.3</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?</H4> lead to a race condition with other users?</H4>
<P>No. <I>currval()</I> returns the current value assigned by your <P>No. <I>currval()</I> returns the current value assigned by your
backend, not by all users.</P> backend, not by all users.</P>
<H4><A name="4.14.4">4.14.4</A>) Why aren't my sequence numbers <H4><A name="4.13.4">4.13.4</A>) Why aren't my sequence numbers
reused on transaction abort? Why are there gaps in the numbering of reused on transaction abort? Why are there gaps in the numbering of
my sequence/SERIAL column?</H4> my sequence/SERIAL column?</H4>
...@@ -1097,7 +1089,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1097,7 +1089,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
completes. This causes gaps in numbering from aborted completes. This causes gaps in numbering from aborted
transactions.</P> transactions.</P>
<H4><A name="4.15">4.15</A>) What is an <SMALL>OID</SMALL>? What is <H4><A name="4.14">4.14</A>) What is an <SMALL>OID</SMALL>? What is
a <SMALL>TID</SMALL>?</H4> a <SMALL>TID</SMALL>?</H4>
<P>Every row that is created in PostgreSQL gets a unique <P>Every row that is created in PostgreSQL gets a unique
...@@ -1120,7 +1112,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1120,7 +1112,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
are modified or reloaded. They are used by index entries to point are modified or reloaded. They are used by index entries to point
to physical rows.</P> to physical rows.</P>
<H4><A name="4.16">4.16</A>) What is the meaning of some of the <H4><A name="4.15">4.15</A>) What is the meaning of some of the
terms used in PostgreSQL?</H4> terms used in PostgreSQL?</H4>
<P>Some of the source code and older documentation use terms that <P>Some of the source code and older documentation use terms that
...@@ -1149,7 +1141,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1149,7 +1141,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>A list of general database terms can be found at: <A href= <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> "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="4.17">4.17</A>) Why do I get the error <I>"ERROR: <H4><A name="4.16">4.16</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4> Memory exhausted in AllocSetAlloc()"</I>?</H4>
<P>You probably have run out of virtual memory on your system, <P>You probably have run out of virtual memory on your system,
...@@ -1168,12 +1160,12 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1168,12 +1160,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
backend is returning too much data, try it before starting the backend is returning too much data, try it before starting the
client. client.
<H4><A name="4.18">4.18</A>) How do I tell what PostgreSQL version <H4><A name="4.17">4.17</A>) How do I tell what PostgreSQL version
I am running?</H4> I am running?</H4>
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P> <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
<H4><A name="4.19">4.19</A>) Why does my large-object operations <H4><A name="4.18">4.18</A>) Why does my large-object operations
get <I>"invalid large obj descriptor"</I>?</H4> get <I>"invalid large obj descriptor"</I>?</H4>
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE> <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
...@@ -1189,7 +1181,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1189,7 +1181,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
may need to set <CODE>auto-commit off.</CODE></P> may need to set <CODE>auto-commit off.</CODE></P>
<H4><A name="4.20">4.20</A>) How do I create a column that will <H4><A name="4.19">4.19</A>) How do I create a column that will
default to the current time?</H4> default to the current time?</H4>
<P>Use <I>CURRENT_TIMESTAMP</I>:</P> <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
...@@ -1198,7 +1190,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1198,7 +1190,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
</CODE> </CODE>
</PRE> </PRE>
<H4><A name="4.21">4.21</A>) Why are my subqueries using <H4><A name="4.20">4.20</A>) Why are my subqueries using
<CODE><SMALL>IN</SMALL></CODE> so slow?</H4> <CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
<P>In versions prior to 7.4, subqueries were joined to outer queries <P>In versions prior to 7.4, subqueries were joined to outer queries
...@@ -1222,7 +1214,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1222,7 +1214,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
sophisticated join techniques as normal queries, and is prefered sophisticated join techniques as normal queries, and is prefered
to using <CODE>EXISTS</CODE>. to using <CODE>EXISTS</CODE>.
<H4><A name="4.22">4.22</A>) How do I perform an outer join?</H4> <H4><A name="4.21">4.21</A>) How do I perform an outer join?</H4>
<P>PostgreSQL supports outer joins using the SQL standard syntax. <P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P> Here are two examples:</P>
...@@ -1262,7 +1254,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1262,7 +1254,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
ORDER BY col1 ORDER BY col1
</PRE> </PRE>
<H4><A name="4.23">4.23</A>) How do I perform queries using <H4><A name="4.22">4.22</A>) How do I perform queries using
multiple databases?</H4> multiple databases?</H4>
<P>There is no way to query a database other than the current one. <P>There is no way to query a database other than the current one.
...@@ -1274,7 +1266,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1274,7 +1266,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
connections to different databases and merge the results on the connections to different databases and merge the results on the
client side.</P> client side.</P>
<H4><A name="4.24">4.24</A>) How do I return multiple rows or <H4><A name="4.23">4.23</A>) How do I return multiple rows or
columns from a function?</H4> columns from a function?</H4>
<P>In 7.3, you can easily return multiple rows or columns from a <P>In 7.3, you can easily return multiple rows or columns from a
...@@ -1282,7 +1274,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1282,7 +1274,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions"> <a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>. http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
<H4><A name="4.25">4.25</A>) Why can't I reliably create/drop <H4><A name="4.24">4.24</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?</H4> temporary tables in PL/PgSQL functions?</H4>
<P>PL/PgSQL caches function contents, and an unfortunate side effect <P>PL/PgSQL caches function contents, and an unfortunate side effect
is that if a PL/PgSQL function accesses a temporary table, and that is that if a PL/PgSQL function accesses a temporary table, and that
...@@ -1292,7 +1284,7 @@ BYTEA bytea variable-length byte array (null-byte safe) ...@@ -1292,7 +1284,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P> will cause the query to be reparsed every time.</P>
<H4><A name="4.26">4.26</A>) What encryption options are available? <H4><A name="4.25">4.25</A>) What encryption options are available?
</H4> </H4>
<UL> <UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for <LI><I>contrib/pgcrypto</I> contains many encryption functions for
......
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