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| -rw-r--r-- | doc/src/sgml/ddl.sgml | 34 |
1 files changed, 23 insertions, 11 deletions
diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml index 9301f0227d7..b2bab81840c 100644 --- a/doc/src/sgml/ddl.sgml +++ b/doc/src/sgml/ddl.sgml @@ -1301,6 +1301,29 @@ ALTER TABLE products ADD COLUMN description text; value is given (null if you don't specify a <literal>DEFAULT</literal> clause). </para> + <tip> + <para> + From <productname>PostgreSQL</productname> 11, adding a column with + a constant default value no longer means that each row of the table + needs to be updated when the <command>ALTER TABLE</command> statement + is executed. Instead, the default value will be returned the next time + the row is accessed, and applied when the table is rewritten, making + the <command>ALTER TABLE</command> very fast even on large tables. + </para> + + <para> + However, if the default value is volatile (e.g. + <function>clock_timestamp()</function>) + each row will need to be updated with the value calculated at the time + <command>ALTER TABLE</command> is executed. To avoid a potentially + lengthy update operation, particularly if you intend to fill the column + with mostly nondefault values anyway, it may be preferable to add the + column with no default, insert the correct values using + <command>UPDATE</command>, and then add any desired default as described + below. + </para> + </tip> + <para> You can also define constraints on the column at the same time, using the usual syntax: @@ -1315,17 +1338,6 @@ ALTER TABLE products ADD COLUMN description text CHECK (description <> '') correctly. </para> - <tip> - <para> - Adding a column with a default requires updating each row of the - table (to store the new column value). However, if no default is - specified, <productname>PostgreSQL</productname> is able to avoid - the physical update. So if you intend to fill the column with - mostly nondefault values, it's best to add the column with no default, - insert the correct values using <command>UPDATE</command>, and then add any - desired default as described below. - </para> - </tip> </sect2> <sect2 id="ddl-alter-removing-a-column"> |