mirrors/postgres
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Update ALTER TABLE docs to mention using VACUUM FULL for rewrites.

Browse Source 
Remove the claim that ALTER TABLE .. SET DATA TYPE is the fastest way of
rewriting a table, since it no longer is.

Noah Misch and Robert Haas, based on a suggestion from Tom Lane.
This commit is contained in:
Robert Haas 2011-02-04 13:08:56 -05:00
parent 6f59a5e5dd
commit edad08ba54
1 changed files with 8 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

23
doc/src/sgml/ref/alter_table.sgml
View File

@ -403,9 +403,9 @@ ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
for details on the available parameters. Note that the table contents
will not be modified immediately by this command; depending on the
parameter you might need to rewrite the table to get the desired effects.
That can be done with <xref linkend="SQL-CLUSTER">
or one of the forms of <command>ALTER
TABLE</> that forces a table rewrite.
That can be done with <link linkend="SQL-VACUUM">VACUUM
FULL</>, <xref linkend="SQL-CLUSTER"> or one of the forms
of <command>ALTER TABLE</> that forces a table rewrite.
</para>
<note>
@ -777,18 +777,11 @@ ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
</para>
<para>
The fact that <literal>SET DATA 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>
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.
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, such as
SET DATA TYPE. This results in no semantically-visible change in the
table, but gets rid of no-longer-useful data.
</para>
<para>