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Avoid a few more SET DATA TYPE table rewrites.

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When the new type is an unconstrained domain over the old type, we don't
need to rewrite the table.

Noah Misch and Robert Haas
This commit is contained in:
Robert Haas 2011-02-14 23:17:35 -05:00
parent 8e1124eeeb
commit 0d90dc16f8
2 changed files with 24 additions and 11 deletions
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15
doc/src/sgml/ref/alter_table.sgml
View File

@ -766,13 +766,14 @@ 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.
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.
As an exception, if the <literal>USING</> clause does not change the column
contents and the old type is either binary coercible to the new type or
an unconstrained domain over the new type, 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>

20
src/backend/commands/tablecmds.c
View File

@ -6632,10 +6632,14 @@ ATPrepAlterColumnType(List **wqueue,
}
/*
* When the data type of a column is changed, a rewrite might not be require
* if the data type is being changed to its current type, or more interestingly
* to a type to which it is binary coercible. But we must check carefully that
* the USING clause isn't trying to insert some other value.
* When the data type of a column is changed, a rewrite might not be required
* if the new type is sufficiently identical to the old one, and the USING
* clause isn't trying to insert some other value. It's safe to skip the
* rewrite if the old type is binary coercible to the new type, or if the
* new type is an unconstrained domain over the old type. In the case of a
* constrained domain, we could get by with scanning the table and checking
* the constraint rather than actually rewriting it, but we don't currently
* try to do that.
*/
static bool
ATColumnChangeRequiresRewrite(Node *expr, AttrNumber varattno)
@ -6649,6 +6653,14 @@ ATColumnChangeRequiresRewrite(Node *expr, AttrNumber varattno)
return false;
else if (IsA(expr, RelabelType))
expr = (Node *) ((RelabelType *) expr)->arg;
else if (IsA(expr, CoerceToDomain))
{
CoerceToDomain *d = (CoerceToDomain *) expr;
if (GetDomainConstraints(d->resulttype) != NIL)
return true;
expr = (Node *) d->arg;
}
else
return true;
}