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docs: Clarify how ALTER TABLE .. SET COMPRESSION works.

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Justin Pryzby, per a complaint from Michael Paquier. Reviewed by
Dilip Kumar and by me.

Discussion: http://postgr.es/m/20210429040132.GF27406@telsasoft.com
This commit is contained in:
Robert Haas 2021-05-06 08:22:45 -04:00
parent 592f00f8de
commit 2d0f662402
1 changed files with 15 additions and 1 deletions
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16
doc/src/sgml/ref/alter_table.sgml
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@ -391,7 +391,21 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
</term>
<listitem>
<para>
This sets the compression method for a column. The supported compression
This sets the compression method to be used for data inserted into a column.
This does not cause the table to be rewritten, so existing data may still
be compressed with other compression methods. If the table is rewritten with
<command>VACUUM FULL</command> or <command>CLUSTER</command>, or restored
with <application>pg_restore</application>, then all tuples are rewritten
with the configured compression methods.
Also, note that when data is inserted from another relation (for example,
by <command>INSERT ... SELECT</command>), tuples from the source data are
not necessarily detoasted, and any previously compressed data is retained
with its existing compression method, rather than recompressing with the
compression methods of the target columns.
The supported compression
methods are <literal>pglz</literal> and <literal>lz4</literal>.
<literal>lz4</literal> is available only if <literal>--with-lz4</literal>
was used when building <productname>PostgreSQL</productname>.