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

When the timezone is UTC, timestamptz and timestamp are binary coercible
in both directions.  See b8a18ad4850ea5ad7884aa6ab731fd392e73b4ad and
c22ecc6562aac895f0f0529707d7bdb460fd2a49 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

1 files changed, 27 insertions, 5 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 93f13a4778c..59341e2a40f 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -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;
 	}