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Update HISTORY file for 7.0.

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@ -1,9 +1,9 @@
Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Tue Sep 28 01:06:15 EDT 1999
Last updated: Tue Mar 21 16:09:11 EST 2000
Current maintainer: Bruce Momjian (maillist@candle.pha.pa.us)
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
The most recent version of this document can be viewed at the
postgreSQL Web site, http://www.PostgreSQL.org.
@ -87,7 +87,10 @@
regexp searching?
4.14) In a query, how do I detect if a field is NULL?
4.15) What is the difference between the various character types?
4.16) How do I create a serial/auto-incrementing field?
4.16.1) How do I create a serial/auto-incrementing field?
4.16.2) How do I get the value of a serial insert?
4.16.3) Wouldn't use of currval() and nextval() lead to a race
condition with other concurrent backend processes?
4.17) What is an oid? What is a tid?
4.18) What is the meaning of some of the terms used in PostgreSQL?
4.19) Why do I get the error "FATAL: palloc failure: memory
@ -96,6 +99,7 @@
4.21) My large-object operations get invalid large obj descriptor.
Why?
4.22) How do I create a column that will default to the current time?
4.23) Why are my subqueries using IN so slow?
Extending PostgreSQL
@ -144,7 +148,8 @@
PostgreSQL Data Base Management System
Copyright (c) 1994-6 Regents of the University of California
Portions copyright (c) 1996-2000, PostgreSQL, Inc Portions Copyright
(c) 1994-6 Regents of the University of California
Permission to use, copy, modify, and distribute this software and its
documentation for any purpose, without fee, and without a written
@ -198,8 +203,8 @@
Unix/NT porting library. See pgsql/doc/README.NT in the distribution.
There is also a web page at
http://members.tripod.com/~kevlo/postgres/portNT.html. There is
another port using U/Win at http://surya.wipro.com/uwin/ported.html.
http://www.freebsd.org/~kevlo/postgres/portNT.html. There is another
port using U/Win at http://surya.wipro.com/uwin/ported.html.
1.5) Where can I get PostgreSQL?
@ -279,10 +284,14 @@
There is a nice tutorial at http://w3.one.net/~jhoffman/sqltut.htm and
at
http://ourworld.compuserve.com/homepages/Graeme_Birchall/DB2_COOK.HTM.
http://ourworld.compuserve.com/homepages/Graeme_Birchall/HTM_COOK.HTM.
Another one is "Teach Yourself SQL in 21 Days, Second Edition" at
http://members.tripod.com/er4ebus/sql/index.htm
Many of our users like The Practical SQL Handbook, Bowman et al.,
Addison Wesley.
Addison Wesley. Others like Lan Times Guide to SQL, Groff et al.,
Osborne McGraw-Hill.
1.11) Is PostgreSQL Y2K compliant?
@ -295,7 +304,7 @@
subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third,
submit high-quality patches to pgsql-patches.
There are about a dozen people who have commit privileges to the
There are about a dozen people who have COMMIT privileges to the
PostgreSQL CVS archive. All of them have submitted so many
high-quality patches that it was a pain for the existing committers to
keep up, and we had confidence that patches they committed were likely
@ -506,13 +515,13 @@
3.10) How do I tune the database engine for better performance?
Certainly, indices can speed up queries. The explain command allows
Certainly, indices can speed up queries. The EXPLAIN command allows
you to see how PostgreSQL is interpreting your query, and which
indices are being used.
If you are doing a lot of inserts, consider doing them in a large
batch using the copy command. This is much faster than single
individual inserts. Second, statements not in a begin work/commit
If you are doing a lot of INSERTs, consider doing them in a large
batch using the COPY command. This is much faster than single
individual INSERTS. Second, statements not in a BEGIN WORK/COMMIT
transaction block are considered to be in their own transaction.
Consider performing several statements in a single transaction block.
This reduces the transaction overhead. Also consider dropping and
@ -534,7 +543,7 @@
is unwise to make this value too large, or you may run out of memory
when a query invokes several concurrent sorts.
You can also use the cluster command to group data in base tables to
You can also use the CLUSTER command to group data in base tables to
match an index. See the cluster(l) manual page for more details.
3.11) What debugging features are available in PostgreSQL?
@ -607,7 +616,7 @@
3.13) What are the pg_tempNNN.NN files in my database directory?
They are temporary files generated by the query executor. For example,
if a sort needs to be done to satisfy an order by, and the sort
if a sort needs to be done to satisfy an ORDER BY, and the sort
requires more space than the backend's -S parameter allows, then temp
files are created to hold the extra data.
@ -648,16 +657,17 @@
4.2) What is the exact difference between binary cursors and normal
cursors?
See the declare manual page for a description.
See the DECLARE manual page for a description.
4.3) How do I select only the first few rows of a query?
4.3) How do I SELECT only the first few rows of a query?
See the fetch manual page, or use SELECT ... LIMIT....
See the FETCH manual page, or use SELECT ... LIMIT....
This only prevents all row results from being transferred to the
client. The entire query must be evaluated, even if you only want just
the first few rows. Consider a query that has an order by. There is no
way to return any rows until the entire query is evaluated and sorted.
The entire query may have to be evaluated, even if you only want the
first few rows. Consider a query that has an ORDER BY. If there is an
index that matches the ORDER BY, PostgreSQL may be able to evaluate
only the first few records requested, or the entire query may have to
be evaluated until the desired rows have been generated.
4.4) How do I get a list of tables, or other information I see in psql?
@ -669,7 +679,7 @@
4.5) How do you remove a column from a table?
We do not support alter table drop column, but do this:
We do not support ALTER TABLE DROP COLUMN, but do this:
SELECT ... -- select all columns but the one you want to remove
INTO TABLE new_table
FROM old_table;
@ -725,35 +735,36 @@
\? to see them.
Also try the file pgsql/src/tutorial/syscat.source. It illustrates
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.
4.9) My queries are slow or don't make use of the indexes. Why?
PostgreSQL does not automatically maintain statistics. One has to make
an explicit vacuum call to update the statistics. After statistics are
an explicit VACUUM call to update the statistics. After statistics are
updated, the optimizer knows how many rows in the table, and can
better decide if it should use indices. Note that the optimizer does
not use indices in cases when the table is small because a sequential
scan would be faster.
For column-specific optimization statistics, use vacuum analyze.
Vacuum analyze is important for complex multi-join queries, so the
For column-specific optimization statistics, use VACUUM ANALYZE.
VACUUM ANALYZE is important for complex multi-join queries, so the
optimizer can estimate the number of rows returned from each table,
and choose the proper join order. The backend does not keep track of
column statistics on its own, so vacuum analyze must be run to collect
column statistics on its own, so VACUUM ANALYZE must be run to collect
them periodically.
Indexes are not used for order by operations.
Indexes are not used for ORDER BY operations.
When using wild-card operators such as LIKE or ~, indices can only be
used if the beginning of the search is anchored to the start of the
string. So, to use indices, LIKE searches can should not begin with %,
and ~(regular expression searches) should start with ^.
and ~(regular expression searches) should start with ^. If you have
locale enabled, indexes can't be used for wild-card searches.
4.10) How do I see how the query optimizer is evaluating my query?
See the explain manual page.
See the EXPLAIN manual page.
4.11) What is an R-tree index?
@ -816,14 +827,60 @@ BYTEA bytea variable-length array of bytes
using them. Specifically, the penalty is for access to all columns
after the first column of this type.
4.16) How do I create a serial/auto-incrementing field?
4.16.1) How do I create a serial/auto-incrementing field?
PostgreSQL supports a serial data type. It auto-creates a sequence and
index on the column. See the create_sequence manual page for more
information about sequences. You can also use each row's oid field as
a unique value. 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.
PostgreSQL supports SERIAL data type. It auto-creates a sequence and
index on the column. For example, this...
CREATE TABLE person (
id SERIAL,
name TEXT
);
...is automatically translated into this...
CREATE SEQUENCE person_id_seq;
CREATE TABLE person (
id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
name TEXT
);
CREATE UNIQUE INDEX person_id_key ON person ( id );
See the create_sequence manual page for more information about
sequences. You can also use each row's oid field as a unique value.
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.
For more details, see Bruce Momjian's chapter on Numbering Rows.
4.16.2) How do I get the back the generated SERIAL value after an insert?
Probably the simplest approach is to to retrieve the next SERIAL value
from the sequence object with the nextval() function before inserting
and then insert it explicitly. Using the example table in 4.16.1, that
might look like this:
$newSerialID = nextval('person_id_seq');
INSERT INTO person (id, name) VALUES ($newSerialID, 'Blaise Pascal');
You would then also have the new value stored in $newSerialID for use
in other queries (e.g., as a foreign key to the person table). Note
that the name of the automatically-created SEQUENCE object will be
named <table>_<serialcolumn>_seq, where table and serialcolumn are the
names of your table and your SERIAL column, respectively.
Similarly, you could retrieve the just-assigned SERIAL value with the
currval() function after it was inserted by default, e.g.,
INSERT INTO person (name) VALUES ('Blaise Pascal');
$newID = currval('person_id_seq');
Finally, you could use the oid returned from the INSERT statement to
lookup the default value, though this is probably the least portable
approach. In perl, using DBI with Edmund Mergl's DBD::Pg module, the
oid value is made available via $sth->{pg_oid_status} after
$sth->execute().
4.16.3) Wouldn't use of currval() and nextval() lead to a race condition
with other concurrent backend processes?
No. That has been handled by the backends.
4.17) What is an oid? What is a tid?
@ -858,9 +915,9 @@ BYTEA bytea variable-length array of bytes
Some of the source code and older documentation use terms that have
more common usage. Here are some:
* table, relation, class
* row, record, tuple
* attribute, field, column
* table, class
* column, field, attribute
* retrieve, select
* replace, update
* append, insert
@ -913,10 +970,27 @@ BYTEA bytea variable-length array of bytes
but this makes the column default to the time of table creation, not
the time of row insertion. Instead do:
create table test (x int, modtime timestamp default text 'now');
CREATE TABLE test (x int, modtime timestamp default now() );
The casting of the value to text prevents the default value from being
computed at table creation time, and delays it until insertion time.
The calling of the function now() prevents the default value from
being computed at table creation time, and delays it until insertion
time. We believe this will not be a problem in post-6.5.* releases.
4.23) Why are my subqueries using IN so slow?
Currently, we join subqueries to outer queries by sequential scanning
the result of the subquery for each row of the outer query. A
workaround is to replace IN with EXISTS. For example, change:
SELECT *
FROM tab
WHERE col1 IN (SELECT col2 FROM TAB2)
to:
SELECT *
FROM tab
WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
We hope to fix this limitation in a future release.
_________________________________________________________________
Extending PostgreSQL