diff options
| -rw-r--r-- | doc/src/sgml/ref/alter_table.sgml | 25 |
1 files changed, 17 insertions, 8 deletions
diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml index 6119a150626..e86712e26b5 100644 --- a/doc/src/sgml/ref/alter_table.sgml +++ b/doc/src/sgml/ref/alter_table.sgml @@ -1,5 +1,5 @@ <!-- -$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.69 2004/05/05 04:48:45 tgl Exp $ +$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.70 2004/05/27 03:30:11 tgl Exp $ PostgreSQL documentation --> @@ -399,17 +399,26 @@ where <replaceable class="PARAMETER">action</replaceable> is one of: <para> The <literal>DROP COLUMN</literal> form does not physically remove the column, but simply makes it invisible to SQL operations. Subsequent - insert and update operations in the table will store a null value for the column. - Thus, dropping a column is quick but it will not immediately reduce the - on-disk size of your table, as the space occupied + insert and update operations in the table will store a null value for the + column. Thus, dropping a column is quick but it will not immediately + reduce the on-disk size of your table, as the space occupied by the dropped column is not reclaimed. The space will be reclaimed over time as existing rows are updated. - To reclaim the space at once, do a dummy <command>UPDATE</> of all rows - and then vacuum, as in: + </para> + + <para> + The fact that <literal>ALTER TYPE</> requires rewriting the whole table + is sometimes an advantage, because the rewriting process eliminates + any dead space in the table. For example, to reclaim the space occupied + by a dropped column immediately, the fastest way is <programlisting> -UPDATE table SET col = col; -VACUUM FULL table; +ALTER TABLE table ALTER COLUMN anycol TYPE anytype; </programlisting> + where <literal>anycol</> is any remaining table column and + <literal>anytype</> is the same type that column already has. + This results in no semantically-visible change in the table, + but the command forces rewriting, which gets rid of no-longer-useful + data. </para> <para> |