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Teach ALTER TABLE .. SET DATA TYPE to avoid some table rewrites.

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When the old type is binary coercible to the new type and the using
clause does not change the column contents, we can avoid a full table
rewrite, though any indexes on the affected columns will still need
to be rebuilt.  This applies, for example, when changing a varchar
column to be of type text.

The prior coding assumed that the set of operations that force a
rewrite is identical to the set of operations that must be propagated
to tables making use of the affected table's rowtype.  This is
no longer true: even though the tuples in those tables wouldn't
need to be modified, the data type change invalidate indexes built
using those composite type columns.  Indexes on the table we're
actually modifying can be invalidated too, of course, but the
existing machinery is sufficient to handle that case.

Along the way, add some debugging messages that make it possible
to understand what operations ALTER TABLE is actually performing
in these cases.

Noah Misch and Robert Haas
This commit is contained in:
Robert Haas
2011-02-12 08:27:55 -05:00
parent 24d1280c4d
commit d31e2a495b
3 changed files with 83 additions and 28 deletions
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16
doc/src/sgml/ref/alter_table.sgml
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@ -766,9 +766,13 @@ ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
<para>
Adding a column with a non-null default or changing the type of an
existing column will require the entire table and indexes to be rewritten.
This might take a significant amount of time for a large table; and it will
temporarily require double the disk space. Adding or removing a system
<literal>oid</> column likewise requires rewriting the entire table.
As an exception, if the old type type is binary coercible to the new
type and the <literal>USING</> clause does not change the column contents,
a table rewrite is not needed, but any indexes on the affected columns
must still be rebuilt. Adding or removing a system <literal>oid</> column
also requires rewriting the entire table. Table and/or index rebuilds may
take a significant amount of time for a large table; and will temporarily
require as much as double the disk space.
</para>
<para>
@ -797,9 +801,9 @@ ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
<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, such as
SET DATA TYPE. This results in no semantically-visible change in the
table, but gets rid of no-longer-useful data.
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.
</para>
<para>