diff options
Diffstat (limited to 'doc/src/sgml/ref/alter_table.sgml')
| -rw-r--r-- | doc/src/sgml/ref/alter_table.sgml | 21 |
1 files changed, 14 insertions, 7 deletions
diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml index 207fec1758e..d0095832559 100644 --- a/doc/src/sgml/ref/alter_table.sgml +++ b/doc/src/sgml/ref/alter_table.sgml @@ -646,7 +646,7 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable> This form changes the information which is written to the write-ahead log to identify rows which are updated or deleted. This option has no effect except when logical replication is in use. <literal>DEFAULT</> - (the default for non-system tables) records the + (the default for non-system tables) records the old values of the columns of the primary key, if any. <literal>USING INDEX</> records the old values of the columns covered by the named index, which must be unique, not partial, not deferrable, and include only columns marked @@ -948,7 +948,8 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable> <para> Adding a <literal>CHECK</> or <literal>NOT NULL</> constraint requires - scanning the table to verify that existing rows meet the constraint. + scanning the table to verify that existing rows meet the constraint, + but does not require a table rewrite. </para> <para> @@ -970,11 +971,17 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable> </para> <para> - To force an immediate rewrite of the table, you can use - <link linkend="SQL-VACUUM">VACUUM FULL</>, <xref linkend="SQL-CLUSTER"> - or one of the forms of ALTER TABLE that forces a rewrite. This results in - no semantically-visible change in the table, but gets rid of - no-longer-useful data. + To force immediate reclamation of space occupied by a dropped column, + you can execute one of the forms of <command>ALTER TABLE</> that + performs a rewrite of the whole table. This results in reconstructing + each row with the dropped column replaced by a null value. + </para> + + <para> + The rewriting forms of <command>ALTER TABLE</> are not MVCC-safe. + After a table rewrite, the table will appear empty to concurrent + transactions, if they are using a snapshot taken before the rewrite + occurred. See <xref linkend="mvcc-caveats"> for more details. </para> <para> |