Rewrite interval_hash() so that the hashcodes are equal for values that

interval_eq() considers equal.  I'm not sure how that fundamental requirement
escaped us through multiple revisions of this hash function, but there it is;
it's been wrong since interval_hash was first written for PG 7.1.
Per bug #4748 from Roman Kononov.

Backpatch to all supported releases.

This patch changes the contents of hash indexes for interval columns.  That's
no particular problem for PG 8.4, since we've broken on-disk compatibility
of hash indexes already; but it will require a migration warning note in
the next minor releases of all existing branches: "if you have any hash
indexes on columns of type interval, REINDEX them after updating".

src/backend/utils/adt/timestamp.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.197 2009/03/15 20:31:19 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.198 2009/04/04 04:53:25 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2041,26 +2041,29 @@ timestamptz_cmp_timestamp(PG_FUNCTION_ARGS)
  *
  *		collate invalid interval at the end
  */
+static inline TimeOffset
+interval_cmp_value(const Interval *interval)
+{
+	TimeOffset	span;
+
+	span = interval->time;
+
+#ifdef HAVE_INT64_TIMESTAMP
+	span += interval->month * INT64CONST(30) * USECS_PER_DAY;
+	span += interval->day * INT64CONST(24) * USECS_PER_HOUR;
+#else
+	span += interval->month * ((double) DAYS_PER_MONTH * SECS_PER_DAY);
+	span += interval->day * ((double) HOURS_PER_DAY * SECS_PER_HOUR);
+#endif
+
+	return span;
+}
+
 static int
 interval_cmp_internal(Interval *interval1, Interval *interval2)
 {
-	TimeOffset	span1,
-				span2;
-
-	span1 = interval1->time;
-	span2 = interval2->time;
-
-#ifdef HAVE_INT64_TIMESTAMP
-	span1 += interval1->month * INT64CONST(30) * USECS_PER_DAY;
-	span1 += interval1->day * INT64CONST(24) * USECS_PER_HOUR;
-	span2 += interval2->month * INT64CONST(30) * USECS_PER_DAY;
-	span2 += interval2->day * INT64CONST(24) * USECS_PER_HOUR;
-#else
-	span1 += interval1->month * ((double) DAYS_PER_MONTH * SECS_PER_DAY);
-	span1 += interval1->day * ((double) HOURS_PER_DAY * SECS_PER_HOUR);
-	span2 += interval2->month * ((double) DAYS_PER_MONTH * SECS_PER_DAY);
-	span2 += interval2->day * ((double) HOURS_PER_DAY * SECS_PER_HOUR);
-#endif
+	TimeOffset	span1 = interval_cmp_value(interval1);
+	TimeOffset	span2 = interval_cmp_value(interval2);
 
 	return ((span1 < span2) ? -1 : (span1 > span2) ? 1 : 0);
 }
@@ -2128,32 +2131,24 @@ interval_cmp(PG_FUNCTION_ARGS)
 	PG_RETURN_INT32(interval_cmp_internal(interval1, interval2));
 }
 
+/*
+ * Hashing for intervals
+ *
+ * We must produce equal hashvals for values that interval_cmp_internal()
+ * considers equal.  So, compute the net span the same way it does,
+ * and then hash that, using either int64 or float8 hashing.
+ */
 Datum
 interval_hash(PG_FUNCTION_ARGS)
 {
-	Interval   *key = PG_GETARG_INTERVAL_P(0);
-	uint32		thash;
-	uint32		mhash;
+	Interval   *interval = PG_GETARG_INTERVAL_P(0);
+	TimeOffset	span = interval_cmp_value(interval);
 
-	/*
-	 * To avoid any problems with padding bytes in the struct, we figure the
-	 * field hashes separately and XOR them.  This also provides a convenient
-	 * framework for dealing with the fact that the time field might be either
-	 * double or int64.
-	 */
 #ifdef HAVE_INT64_TIMESTAMP
-	thash = DatumGetUInt32(DirectFunctionCall1(hashint8,
-											   Int64GetDatumFast(key->time)));
+	return DirectFunctionCall1(hashint8, Int64GetDatumFast(span));
 #else
-	thash = DatumGetUInt32(DirectFunctionCall1(hashfloat8,
-											 Float8GetDatumFast(key->time)));
+	return DirectFunctionCall1(hashfloat8, Float8GetDatumFast(span));
 #endif
-	thash ^= DatumGetUInt32(hash_uint32(key->day));
-	/* Shift so "k days" and "k months" don't hash to the same thing */
-	mhash = DatumGetUInt32(hash_uint32(key->month));
-	thash ^= mhash << 24;
-	thash ^= mhash >> 8;
-	PG_RETURN_UINT32(thash);
 }
 
 /* overlaps_timestamp() --- implements the SQL92 OVERLAPS operator.

src/test/regress/expected/interval.out

@@ -717,3 +717,16 @@ select interval '0:0:0.7', interval '@ 0.70 secs', interval '0.7 seconds';
  @ 0.7 secs | @ 0.7 secs | @ 0.7 secs
 (1 row)
 
+-- check that '30 days' equals '1 month' according to the hash function
+select '30 days'::interval = '1 month'::interval as t;
+ t 
+---
+ t
+(1 row)
+
+select interval_hash('30 days'::interval) = interval_hash('1 month'::interval) as t;
+ t 
+---
+ t
+(1 row)
+

src/test/regress/sql/interval.sql

@@ -241,3 +241,7 @@ SET IntervalStyle to postgres_verbose;
 select interval '-10 mons -3 days +03:55:06.70';
 select interval '1 year 2 mons 3 days 04:05:06.699999';
 select interval '0:0:0.7', interval '@ 0.70 secs', interval '0.7 seconds'; 
+
+-- check that '30 days' equals '1 month' according to the hash function
+select '30 days'::interval = '1 month'::interval as t;
+select interval_hash('30 days'::interval) = interval_hash('1 month'::interval) as t;