ALTER TABLE: skip FK validation when it's safe to do so

We already skip rewriting the table in these cases, but we still force a
whole table scan to validate the data.  This can be skipped, and thus
we can make the whole ALTER TABLE operation just do some catalog touches
instead of scanning the table, when these two conditions hold:

(a) Old and new pg_constraint.conpfeqop match exactly.  This is actually
stronger than needed; we could loosen things by way of operator
families, but it'd require a lot more effort.

(b) The functions, if any, implementing a cast from the foreign type to
the primary opcintype are the same.  For this purpose, we can consider a
binary coercion equivalent to an exact type match.  When the opcintype
is polymorphic, require that the old and new foreign types match
exactly.  (Since ri_triggers.c does use the executor, the stronger check
for polymorphic types is no mere future-proofing.  However, no core type
exercises its necessity.)

Author: Noah Misch

Committer's note: catalog version bumped due to change of the Constraint
node.  I can't actually find any way to have such a node in a stored
rule, but given that we have "out" support for them, better be safe.

src/backend/commands/tablecmds.c

@@ -276,6 +276,8 @@ static Oid transformFkeyCheckAttrs(Relation pkrel,
 						int numattrs, int16 *attnums,
 						Oid *opclasses);
 static void checkFkeyPermissions(Relation rel, int16 *attnums, int natts);
+static CoercionPathType findFkeyCast(Oid targetTypeId, Oid sourceTypeId,
+			 Oid *funcid);
 static void validateCheckConstraint(Relation rel, HeapTuple constrtup);
 static void validateForeignKeyConstraint(char *conname,
 							 Relation rel, Relation pkrel,
@@ -358,6 +360,7 @@ static void ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMOD
 static void ATPostAlterTypeParse(Oid oldId, char *cmd,
 					 List **wqueue, LOCKMODE lockmode, bool rewrite);
 static void TryReuseIndex(Oid oldId, IndexStmt *stmt);
+static void TryReuseForeignKey(Oid oldId, Constraint *con);
 static void change_owner_fix_column_acls(Oid relationOid,
 							 Oid oldOwnerId, Oid newOwnerId);
 static void change_owner_recurse_to_sequences(Oid relationOid,
@@ -5620,6 +5623,8 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 				numpks;
 	Oid			indexOid;
 	Oid			constrOid;
+	bool		old_check_ok;
+	ListCell   *old_pfeqop_item = list_head(fkconstraint->old_conpfeqop);
 
 	/*
 	 * Grab an exclusive lock on the pk table, so that someone doesn't delete
@@ -5736,6 +5741,13 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 				(errcode(ERRCODE_INVALID_FOREIGN_KEY),
 				 errmsg("number of referencing and referenced columns for foreign key disagree")));
 
+	/*
+	 * On the strength of a previous constraint, we might avoid scanning
+	 * tables to validate this one.  See below.
+	 */
+	old_check_ok = (fkconstraint->old_conpfeqop != NIL);
+	Assert(!old_check_ok || numfks == list_length(fkconstraint->old_conpfeqop));
+
 	for (i = 0; i < numpks; i++)
 	{
 		Oid			pktype = pktypoid[i];
@@ -5750,6 +5762,7 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 		Oid			ppeqop;
 		Oid			ffeqop;
 		int16		eqstrategy;
+		Oid			pfeqop_right;
 
 		/* We need several fields out of the pg_opclass entry */
 		cla_ht = SearchSysCache1(CLAOID, ObjectIdGetDatum(opclasses[i]));
@@ -5792,10 +5805,17 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 		pfeqop = get_opfamily_member(opfamily, opcintype, fktyped,
 									 eqstrategy);
 		if (OidIsValid(pfeqop))
+		{
+			pfeqop_right = fktyped;
 			ffeqop = get_opfamily_member(opfamily, fktyped, fktyped,
 										 eqstrategy);
+		}
 		else
-			ffeqop = InvalidOid;	/* keep compiler quiet */
+		{
+			/* keep compiler quiet */
+			pfeqop_right = InvalidOid;
+			ffeqop = InvalidOid;
+		}
 
 		if (!(OidIsValid(pfeqop) && OidIsValid(ffeqop)))
 		{
@@ -5817,7 +5837,10 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 			target_typeids[1] = opcintype;
 			if (can_coerce_type(2, input_typeids, target_typeids,
 								COERCION_IMPLICIT))
+			{
 				pfeqop = ffeqop = ppeqop;
+				pfeqop_right = opcintype;
+			}
 		}
 
 		if (!(OidIsValid(pfeqop) && OidIsValid(ffeqop)))
@@ -5833,6 +5856,77 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 							   format_type_be(fktype),
 							   format_type_be(pktype))));
 
+		if (old_check_ok)
+		{
+			/*
+			 * When a pfeqop changes, revalidate the constraint.  We could
+			 * permit intra-opfamily changes, but that adds subtle complexity
+			 * without any concrete benefit for core types.  We need not
+			 * assess ppeqop or ffeqop, which RI_Initial_Check() does not use.
+			 */
+			old_check_ok = (pfeqop == lfirst_oid(old_pfeqop_item));
+			old_pfeqop_item = lnext(old_pfeqop_item);
+		}
+		if (old_check_ok)
+		{
+			Oid			old_fktype;
+			Oid			new_fktype;
+			CoercionPathType old_pathtype;
+			CoercionPathType new_pathtype;
+			Oid			old_castfunc;
+			Oid			new_castfunc;
+
+			/*
+			 * Identify coercion pathways from each of the old and new FK-side
+			 * column types to the right (foreign) operand type of the pfeqop.
+			 * We may assume that pg_constraint.conkey is not changing.
+			 */
+			old_fktype = tab->oldDesc->attrs[fkattnum[i] - 1]->atttypid;
+			new_fktype = fktype;
+			old_pathtype = findFkeyCast(pfeqop_right, old_fktype,
+										&old_castfunc);
+			new_pathtype = findFkeyCast(pfeqop_right, new_fktype,
+										&new_castfunc);
+
+			/*
+			 * Upon a change to the cast from the FK column to its pfeqop
+			 * operand, revalidate the constraint.  For this evaluation, a
+			 * binary coercion cast is equivalent to no cast at all.  While
+			 * type implementors should design implicit casts with an eye
+			 * toward consistency of operations like equality, we cannot assume
+			 * here that they have done so.
+			 *
+			 * A function with a polymorphic argument could change behavior
+			 * arbitrarily in response to get_fn_expr_argtype().  Therefore,
+			 * when the cast destination is polymorphic, we only avoid
+			 * revalidation if the input type has not changed at all.  Given
+			 * just the core data types and operator classes, this requirement
+			 * prevents no would-be optimizations.
+			 *
+			 * If the cast converts from a base type to a domain thereon, then
+			 * that domain type must be the opcintype of the unique index.
+			 * Necessarily, the primary key column must then be of the domain
+			 * type.  Since the constraint was previously valid, all values on
+			 * the foreign side necessarily exist on the primary side and in
+			 * turn conform to the domain.  Consequently, we need not treat
+			 * domains specially here.
+			 *
+			 * Since we require that all collations share the same notion of
+			 * equality (which they do, because texteq reduces to bitwise
+			 * equality), we don't compare collation here.
+			 *
+			 * We need not directly consider the PK type.  It's necessarily
+			 * binary coercible to the opcintype of the unique index column,
+			 * and ri_triggers.c will only deal with PK datums in terms of that
+			 * opcintype.  Changing the opcintype also changes pfeqop.
+			 */
+			old_check_ok = (new_pathtype == old_pathtype &&
+							new_castfunc == old_castfunc &&
+							(!IsPolymorphicType(pfeqop_right) ||
+							 new_fktype == old_fktype));
+
+		}
+
 		pfeqoperators[i] = pfeqop;
 		ppeqoperators[i] = ppeqop;
 		ffeqoperators[i] = ffeqop;
@@ -5877,10 +5971,12 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 
 	/*
 	 * Tell Phase 3 to check that the constraint is satisfied by existing rows.
-	 * We can skip this during table creation, or if requested explicitly by
-	 * specifying NOT VALID in an ADD FOREIGN KEY command.
+	 * We can skip this during table creation, when requested explicitly by
+	 * specifying NOT VALID in an ADD FOREIGN KEY command, and when we're
+	 * recreating a constraint following a SET DATA TYPE operation that did not
+	 * impugn its validity.
 	 */
-	if (!fkconstraint->skip_validation)
+	if (!old_check_ok && !fkconstraint->skip_validation)
 	{
 		NewConstraint *newcon;
 
@@ -6330,6 +6426,35 @@ transformFkeyCheckAttrs(Relation pkrel,
 	return indexoid;
 }
 
+/*
+ * findFkeyCast -
+ *
+ *	Wrapper around find_coercion_pathway() for ATAddForeignKeyConstraint().
+ *	Caller has equal regard for binary coercibility and for an exact match.
+*/
+static CoercionPathType
+findFkeyCast(Oid targetTypeId, Oid sourceTypeId, Oid *funcid)
+{
+	CoercionPathType ret;
+
+	if (targetTypeId == sourceTypeId)
+	{
+		ret = COERCION_PATH_RELABELTYPE;
+		*funcid = InvalidOid;
+	}
+	else
+	{
+		ret = find_coercion_pathway(targetTypeId, sourceTypeId,
+									COERCION_IMPLICIT, funcid);
+		if (ret == COERCION_PATH_NONE)
+			/* A previously-relied-upon cast is now gone. */
+			elog(ERROR, "could not find cast from %u to %u",
+				 sourceTypeId, targetTypeId);
+	}
+
+	return ret;
+}
+
 /* Permissions checks for ADD FOREIGN KEY */
 static void
 checkFkeyPermissions(Relation rel, int16 *attnums, int natts)
@@ -7717,6 +7842,7 @@ ATPostAlterTypeParse(Oid oldId, char *cmd,
 					foreach(lcmd, stmt->cmds)
 					{
 						AlterTableCmd *cmd = (AlterTableCmd *) lfirst(lcmd);
+						Constraint *con;
 
 						switch (cmd->subtype)
 						{
@@ -7730,6 +7856,12 @@ ATPostAlterTypeParse(Oid oldId, char *cmd,
 									lappend(tab->subcmds[AT_PASS_OLD_INDEX], cmd);
 								break;
 							case AT_AddConstraint:
+								Assert(IsA(cmd->def, Constraint));
+								con = (Constraint *) cmd->def;
+								/* rewriting neither side of a FK */
+								if (con->contype == CONSTR_FOREIGN &&
+									!rewrite && !tab->rewrite)
+									TryReuseForeignKey(oldId, con);
 								tab->subcmds[AT_PASS_OLD_CONSTR] =
 									lappend(tab->subcmds[AT_PASS_OLD_CONSTR], cmd);
 								break;
@@ -7768,6 +7900,50 @@ TryReuseIndex(Oid oldId, IndexStmt *stmt)
 	}
 }
 
+/*
+ * Subroutine for ATPostAlterTypeParse().
+ *
+ * Stash the old P-F equality operator into the Constraint node, for possible
+ * use by ATAddForeignKeyConstraint() in determining whether revalidation of
+ * this constraint can be skipped.
+ */
+static void
+TryReuseForeignKey(Oid oldId, Constraint *con)
+{
+	HeapTuple	tup;
+	Datum		adatum;
+	bool		isNull;
+	ArrayType  *arr;
+	Oid		   *rawarr;
+	int			numkeys;
+	int			i;
+
+	Assert(con->contype == CONSTR_FOREIGN);
+	Assert(con->old_conpfeqop == NIL); /* already prepared this node */
+
+	tup = SearchSysCache1(CONSTROID, ObjectIdGetDatum(oldId));
+	if (!HeapTupleIsValid(tup)) /* should not happen */
+		elog(ERROR, "cache lookup failed for constraint %u", oldId);
+
+	adatum = SysCacheGetAttr(CONSTROID, tup,
+							 Anum_pg_constraint_conpfeqop, &isNull);
+	if (isNull)
+		elog(ERROR, "null conpfeqop for constraint %u", oldId);
+	arr = DatumGetArrayTypeP(adatum);	/* ensure not toasted */
+	numkeys = ARR_DIMS(arr)[0];
+	/* test follows the one in ri_FetchConstraintInfo() */
+	if (ARR_NDIM(arr) != 1 ||
+		ARR_HASNULL(arr) ||
+		ARR_ELEMTYPE(arr) != OIDOID)
+		elog(ERROR, "conpfeqop is not a 1-D Oid array");
+	rawarr = (Oid *) ARR_DATA_PTR(arr);
+
+	/* stash a List of the operator Oids in our Constraint node */
+	for (i = 0; i < numkeys; i++)
+		con->old_conpfeqop = lcons_oid(rawarr[i], con->old_conpfeqop);
+
+	ReleaseSysCache(tup);
+}
 
 /*
  * ALTER TABLE OWNER

src/backend/nodes/copyfuncs.c

@@ -2364,6 +2364,7 @@ _copyConstraint(const Constraint *from)
 	COPY_SCALAR_FIELD(fk_matchtype);
 	COPY_SCALAR_FIELD(fk_upd_action);
 	COPY_SCALAR_FIELD(fk_del_action);
+	COPY_NODE_FIELD(old_conpfeqop);
 	COPY_SCALAR_FIELD(skip_validation);
 	COPY_SCALAR_FIELD(initially_valid);
 

src/backend/nodes/equalfuncs.c

@@ -2199,6 +2199,7 @@ _equalConstraint(const Constraint *a, const Constraint *b)
 	COMPARE_SCALAR_FIELD(fk_matchtype);
 	COMPARE_SCALAR_FIELD(fk_upd_action);
 	COMPARE_SCALAR_FIELD(fk_del_action);
+	COMPARE_NODE_FIELD(old_conpfeqop);
 	COMPARE_SCALAR_FIELD(skip_validation);
 	COMPARE_SCALAR_FIELD(initially_valid);
 

src/backend/nodes/outfuncs.c

@@ -2626,6 +2626,7 @@ _outConstraint(StringInfo str, const Constraint *node)
 			WRITE_CHAR_FIELD(fk_matchtype);
 			WRITE_CHAR_FIELD(fk_upd_action);
 			WRITE_CHAR_FIELD(fk_del_action);
+			WRITE_NODE_FIELD(old_conpfeqop);
 			WRITE_BOOL_FIELD(skip_validation);
 			WRITE_BOOL_FIELD(initially_valid);
 			break;

src/backend/utils/adt/ri_triggers.c

@@ -3224,6 +3224,7 @@ ri_FetchConstraintInfo(RI_ConstraintInfo *riinfo,
 		elog(ERROR, "null conpfeqop for constraint %u", constraintOid);
 	arr = DatumGetArrayTypeP(adatum);	/* ensure not toasted */
 	numkeys = ARR_DIMS(arr)[0];
+	/* see TryReuseForeignKey if you change the test below */
 	if (ARR_NDIM(arr) != 1 ||
 		numkeys != riinfo->nkeys ||
 		numkeys > RI_MAX_NUMKEYS ||

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201202191
+#define CATALOG_VERSION_NO	201202271
 
 #endif

src/include/nodes/parsenodes.h

@@ -1552,6 +1552,7 @@ typedef struct Constraint
 	char		fk_matchtype;	/* FULL, PARTIAL, UNSPECIFIED */
 	char		fk_upd_action;	/* ON UPDATE action */
 	char		fk_del_action;	/* ON DELETE action */
+	List	   *old_conpfeqop;	/* pg_constraint.conpfeqop of my former self */
 
 	/* Fields used for constraints that allow a NOT VALID specification */
 	bool		skip_validation;	/* skip validation of existing rows? */