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Avoid some table rewrites for ALTER TABLE .. SET DATA TYPE timestamp.

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When the timezone is UTC, timestamptz and timestamp are binary coercible
in both directions.  See b8a18ad485 and
c22ecc6562 for the previous attempt in
this problem space.  Skip the table rewrite; for now, continue to
needlessly rewrite any index on an affected column.

Reviewed by Simon Riggs and Tom Lane.

Discussion: https://postgr.es/m/20190226061450.GA1665944@rfd.leadboat.com
This commit is contained in:
Noah Misch
2019-03-08 20:16:27 -08:00
parent 82a5649fb9
commit 3c5926301a
5 changed files with 65 additions and 7 deletions
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32
src/backend/commands/tablecmds.c
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@ -96,6 +96,7 @@
#include "utils/ruleutils.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"
#include "utils/timestamp.h"
#include "utils/typcache.h"
@ -9678,11 +9679,15 @@ ATPrepAlterColumnType(List **wqueue,
* 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.
* rewrite in these cases:
*
* - the old type is binary coercible to the new type
* - the new type is an unconstrained domain over the old type
* - {NEW,OLD} or {OLD,NEW} is {timestamptz,timestamp} and the timezone is UTC
*
* 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)
@ -9704,6 +9709,23 @@ ATColumnChangeRequiresRewrite(Node *expr, AttrNumber varattno)
return true;
expr = (Node *) d->arg;
}
else if (IsA(expr, FuncExpr))
{
FuncExpr *f = (FuncExpr *) expr;
switch (f->funcid)
{
case F_TIMESTAMPTZ_TIMESTAMP:
case F_TIMESTAMP_TIMESTAMPTZ:
if (TimestampTimestampTzRequiresRewrite())
return true;
else
expr = linitial(f->args);
break;
default:
return true;
}
}
else
return true;
}