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Update LIMIT/FETCH FAQ item.

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Bruce Momjian
2005-02-01 01:36:13 +00:00
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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Mon Jan 31 19:18:44 EST 2005
Last updated: Mon Jan 31 20:35:58 EST 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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PostgreSQL releases?
The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
However, major releases (e.g. from 7.2 to 7.3) often change the
so upgrading from 7.4 to 7.4.1 does not require a dump and restore.
However, major releases (e.g. from 7.3 to 7.4) often change the
internal format of system tables and data files. These changes are
often complex, so we don't maintain backward compatability for data
often complex, so we don't maintain backward compatibility for data
files. A dump outputs data in a generic format that can then be loaded
in using the new internal format.
In releases where the on-disk format does not change, the pg_upgrade
script can be used to upgrade without a dump/restore. The release
notes mention whether pg_upgrade is available for the release.
3.8) What computer hardware should I use?
Because PC hardware is mostly compatible, people tend to believe that
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4.1) How do I SELECT only the first few rows of a query? A random row?
See the FETCH manual page, or use SELECT ... LIMIT....
The entire query may have to be evaluated, even if you only want the
first few rows. Consider using 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.
To retrieve only a few rows, if you know at the number of rows needed
at the time of the SELECT use LIMIT . If an index matches the ORDER BY
it is possible the entire query does not have to be executed. If you
don't know the number of rows at SELECT time, use a cursor and FETCH.
To SELECT a random row, use:
SELECT col