Separate equalRowTypes() from equalTupleDescs()

This introduces a new function equalRowTypes() that is effectively a
subset of equalTupleDescs() but only compares the number of attributes
and attribute name, type, typmod, and collation.  This is enough for
most existing uses of equalTupleDescs(), which are changed to use the
new function.  The only remaining callers of equalTupleDescs() are
those that really want to check the full tuple descriptor as such,
without concern about record or row or record type semantics.

The existing function hashTupleDesc() is renamed to hashRowType(),
because it now corresponds more to equalRowTypes().

The purpose of this change is to be clearer about the semantics of the
equality asked for by each caller.  (At least one caller had a comment
that questioned whether equalTupleDescs() was too restrictive.)  For
example, 4f622503d6d removed attstattarget from the tuple descriptor
structure.  It was not fully clear at the time how this should affect
equalTupleDescs().  Now the answer is clear: By their own definitions,
equalRowTypes() does not care, and equalTupleDescs() just compares
whatever is in the tuple descriptor but does not care why it is in
there.

Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/f656d6d9-6660-4518-a006-2f65cafbebd1%40eisentraut.org

src/backend/access/common/tupdesc.c

@@ -414,11 +414,6 @@ DecrTupleDescRefCount(TupleDesc tupdesc)
 
 /*
  * Compare two TupleDesc structures for logical equality
- *
- * Note: we deliberately do not check the attrelid and tdtypmod fields.
- * This allows typcache.c to use this routine to see if a cached record type
- * matches a requested type, and is harmless for relcache.c's uses.
- * We don't compare tdrefcount, either.
  */
 bool
 equalTupleDescs(TupleDesc tupdesc1, TupleDesc tupdesc2)
@@ -431,6 +426,8 @@ equalTupleDescs(TupleDesc tupdesc1, TupleDesc tupdesc2)
 	if (tupdesc1->tdtypeid != tupdesc2->tdtypeid)
 		return false;
 
+	/* tdtypmod and tdrefcount are not checked */
+
 	for (i = 0; i < tupdesc1->natts; i++)
 	{
 		Form_pg_attribute attr1 = TupleDescAttr(tupdesc1, i);
@@ -561,17 +558,68 @@ equalTupleDescs(TupleDesc tupdesc1, TupleDesc tupdesc2)
 }
 
 /*
- * hashTupleDesc
- *		Compute a hash value for a tuple descriptor.
+ * equalRowTypes
  *
- * If two tuple descriptors would be considered equal by equalTupleDescs()
+ * This determines whether two tuple descriptors have equal row types.  This
+ * only checks those fields in pg_attribute that are applicable for row types,
+ * while ignoring those fields that define the physical row storage or those
+ * that define table column metadata.
+ *
+ * Specifically, this checks:
+ *
+ * - same number of attributes
+ * - same composite type ID (but could both be zero)
+ * - corresponding attributes (in order) have same the name, type, typmod,
+ *   collation
+ *
+ * This is used to check whether two record types are compatible, whether
+ * function return row types are the same, and other similar situations.
+ *
+ * (XXX There was some discussion whether attndims should be checked here, but
+ * for now it has been decided not to.)
+ *
+ * Note: We deliberately do not check the tdtypmod field.  This allows
+ * typcache.c to use this routine to see if a cached record type matches a
+ * requested type.
+ */
+bool
+equalRowTypes(TupleDesc tupdesc1, TupleDesc tupdesc2)
+{
+	if (tupdesc1->natts != tupdesc2->natts)
+		return false;
+	if (tupdesc1->tdtypeid != tupdesc2->tdtypeid)
+		return false;
+
+	for (int i = 0; i < tupdesc1->natts; i++)
+	{
+		Form_pg_attribute attr1 = TupleDescAttr(tupdesc1, i);
+		Form_pg_attribute attr2 = TupleDescAttr(tupdesc2, i);
+
+		if (strcmp(NameStr(attr1->attname), NameStr(attr2->attname)) != 0)
+			return false;
+		if (attr1->atttypid != attr2->atttypid)
+			return false;
+		if (attr1->atttypmod != attr2->atttypmod)
+			return false;
+		if (attr1->attcollation != attr2->attcollation)
+			return false;
+
+		/* Record types derived from tables could have dropped fields. */
+		if (attr1->attisdropped != attr2->attisdropped)
+			return false;
+	}
+
+	return true;
+}
+
+/*
+ * hashRowType
+ *
+ * If two tuple descriptors would be considered equal by equalRowTypes()
  * then their hash value will be equal according to this function.
- *
- * Note that currently contents of constraint are not hashed - it'd be a bit
- * painful to do so, and conflicts just due to constraints are unlikely.
  */
 uint32
-hashTupleDesc(TupleDesc desc)
+hashRowType(TupleDesc desc)
 {
 	uint32		s;
 	int			i;

src/backend/catalog/pg_proc.c

@@ -437,7 +437,7 @@ ProcedureCreate(const char *procedureName,
 			if (olddesc == NULL && newdesc == NULL)
 				 /* ok, both are runtime-defined RECORDs */ ;
 			else if (olddesc == NULL || newdesc == NULL ||
-					 !equalTupleDescs(olddesc, newdesc))
+					 !equalRowTypes(olddesc, newdesc))
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
 						 errmsg("cannot change return type of existing function"),

src/backend/commands/analyze.c

@@ -1581,7 +1581,7 @@ acquire_inherited_sample_rows(Relation onerel, int elevel,
 
 				/* We may need to convert from child's rowtype to parent's */
 				if (childrows > 0 &&
-					!equalTupleDescs(RelationGetDescr(childrel),
+					!equalRowTypes(RelationGetDescr(childrel),
 								   RelationGetDescr(onerel)))
 				{
 					TupleConversionMap *map;

src/backend/commands/view.c

@@ -30,7 +30,7 @@
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
 
-static void checkViewTupleDesc(TupleDesc newdesc, TupleDesc olddesc);
+static void checkViewColumns(TupleDesc newdesc, TupleDesc olddesc);
 
 /*---------------------------------------------------------------------
  * DefineVirtualRelation
@@ -130,7 +130,7 @@ DefineVirtualRelation(RangeVar *relation, List *tlist, bool replace,
 		 * column list.
 		 */
 		descriptor = BuildDescForRelation(attrList);
-		checkViewTupleDesc(descriptor, rel->rd_att);
+		checkViewColumns(descriptor, rel->rd_att);
 
 		/*
 		 * If new attributes have been added, we must add pg_attribute entries
@@ -258,13 +258,13 @@ DefineVirtualRelation(RangeVar *relation, List *tlist, bool replace,
 }
 
 /*
- * Verify that tupledesc associated with proposed new view definition
- * matches tupledesc of old view.  This is basically a cut-down version
- * of equalTupleDescs(), with code added to generate specific complaints.
- * Also, we allow the new tupledesc to have more columns than the old.
+ * Verify that the columns associated with proposed new view definition match
+ * the columns of the old view.  This is similar to equalRowTypes(), with code
+ * added to generate specific complaints.  Also, we allow the new view to have
+ * more columns than the old.
  */
 static void
-checkViewTupleDesc(TupleDesc newdesc, TupleDesc olddesc)
+checkViewColumns(TupleDesc newdesc, TupleDesc olddesc)
 {
 	int			i;
 

src/backend/utils/cache/plancache.c

@@ -727,8 +727,7 @@ RevalidateCachedQuery(CachedPlanSource *plansource,
 		PopActiveSnapshot();
 
 	/*
-	 * Check or update the result tupdesc.  XXX should we use a weaker
-	 * condition than equalTupleDescs() here?
+	 * Check or update the result tupdesc.
 	 *
 	 * We assume the parameter types didn't change from the first time, so no
 	 * need to update that.
@@ -739,7 +738,7 @@ RevalidateCachedQuery(CachedPlanSource *plansource,
 		/* OK, doesn't return tuples */
 	}
 	else if (resultDesc == NULL || plansource->resultDesc == NULL ||
-			 !equalTupleDescs(resultDesc, plansource->resultDesc))
+			 !equalRowTypes(resultDesc, plansource->resultDesc))
 	{
 		/* can we give a better error message? */
 		if (plansource->fixed_result)

src/backend/utils/cache/typcache.c

@@ -147,7 +147,7 @@ typedef struct TypeCacheEnumData
  * We use a separate table for storing the definitions of non-anonymous
  * record types.  Once defined, a record type will be remembered for the
  * life of the backend.  Subsequent uses of the "same" record type (where
- * sameness means equalTupleDescs) will refer to the existing table entry.
+ * sameness means equalRowTypes) will refer to the existing table entry.
  *
  * Stored record types are remembered in a linear array of TupleDescs,
  * which can be indexed quickly with the assigned typmod.  There is also
@@ -231,7 +231,7 @@ shared_record_table_compare(const void *a, const void *b, size_t size,
 	else
 		t2 = k2->u.local_tupdesc;
 
-	return equalTupleDescs(t1, t2) ? 0 : 1;
+	return equalRowTypes(t1, t2) ? 0 : 1;
 }
 
 /*
@@ -249,7 +249,7 @@ shared_record_table_hash(const void *a, size_t size, void *arg)
 	else
 		t = k->u.local_tupdesc;
 
-	return hashTupleDesc(t);
+	return hashRowType(t);
 }
 
 /* Parameters for SharedRecordTypmodRegistry's TupleDesc table. */
@@ -1927,7 +1927,7 @@ record_type_typmod_hash(const void *data, size_t size)
 {
 	RecordCacheEntry *entry = (RecordCacheEntry *) data;
 
-	return hashTupleDesc(entry->tupdesc);
+	return hashRowType(entry->tupdesc);
 }
 
 /*
@@ -1939,7 +1939,7 @@ record_type_typmod_compare(const void *a, const void *b, size_t size)
 	RecordCacheEntry *left = (RecordCacheEntry *) a;
 	RecordCacheEntry *right = (RecordCacheEntry *) b;
 
-	return equalTupleDescs(left->tupdesc, right->tupdesc) ? 0 : 1;
+	return equalRowTypes(left->tupdesc, right->tupdesc) ? 0 : 1;
 }
 
 /*

src/include/access/tupdesc.h

@@ -126,8 +126,8 @@ extern void DecrTupleDescRefCount(TupleDesc tupdesc);
 	} while (0)
 
 extern bool equalTupleDescs(TupleDesc tupdesc1, TupleDesc tupdesc2);
-
-extern uint32 hashTupleDesc(TupleDesc desc);
+extern bool equalRowTypes(TupleDesc tupdesc1, TupleDesc tupdesc2);
+extern uint32 hashRowType(TupleDesc desc);
 
 extern void TupleDescInitEntry(TupleDesc desc,
 							   AttrNumber attributeNumber,