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| author | Robert Haas <rhaas@postgresql.org> | 2021-05-06 08:22:45 -0400 |
|---|---|---|
| committer | Robert Haas <rhaas@postgresql.org> | 2021-05-06 08:22:45 -0400 |
| commit | 2d0f662402635d591cac9f1daae5e81e7c4374fc (patch) | |
| tree | c321e2be214452511d23a0db506b553af2216692 | |
| parent | 592f00f8dec68038301467a904ac514eddabf6cd (diff) | |
| download | postgresql-2d0f662402635d591cac9f1daae5e81e7c4374fc.tar.gz postgresql-2d0f662402635d591cac9f1daae5e81e7c4374fc.zip | |
docs: Clarify how ALTER TABLE .. SET COMPRESSION works.
Justin Pryzby, per a complaint from Michael Paquier. Reviewed by
Dilip Kumar and by me.
Discussion: http://postgr.es/m/20210429040132.GF27406@telsasoft.com
| -rw-r--r-- | doc/src/sgml/ref/alter_table.sgml | 16 |
1 files changed, 15 insertions, 1 deletions
diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml index 6166b263345..9cef1f101eb 100644 --- a/doc/src/sgml/ref/alter_table.sgml +++ b/doc/src/sgml/ref/alter_table.sgml @@ -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>. |