scbuild: add missing name check for ALTER PRIMARY KEY

Previously, this check was missing, leading the schema change to hang
due to a validation failure in a non-revertible post-commit transaction.

This patch maintains the existing behavior of the legacy schema changer
which doesn't distinguish between unique and non-unique indexes: all
name collisions will be reported as constraint name collisions despite
non-unique indexes not being constraints.

Fixes #88131.

Release justification: low-risk high-value bug fix
Release note: None

pkg/sql/logictest/testdata/logic_test/alter_primary_key

@@ -1763,6 +1763,38 @@ SELECT column_name FROM [SHOW COLUMNS FROM t_rowid] ORDER BY column_name;
 k
 v
 
+subtest check_constraint_name
+
+statement ok
+CREATE TABLE t_name_check (a INT NOT NULL, CONSTRAINT ctcheck CHECK (a > 0))
+
+skipif config local-legacy-schema-changer
+statement error pgcode 42710 constraint with name "ctcheck" already exists
+ALTER TABLE t_name_check ADD CONSTRAINT ctcheck PRIMARY KEY (a)
+
+statement ok
+ALTER TABLE t_name_check ADD CONSTRAINT t_name_check_pkey PRIMARY KEY (a)
+
+statement ok
+DROP TABLE t_name_check
+
+statement ok
+CREATE TABLE t_name_check (a INT NOT NULL, CONSTRAINT ctuniq UNIQUE (a))
+
+skipif config local-legacy-schema-changer
+statement error pgcode 42710 constraint with name "ctuniq" already exists
+ALTER TABLE t_name_check ADD CONSTRAINT ctuniq PRIMARY KEY (a)
+
+statement ok
+DROP TABLE t_name_check
+
+statement ok
+CREATE TABLE t_name_check (a INT NOT NULL, INDEX idx (a))
+
+skipif config local-legacy-schema-changer
+statement error pgcode 42710 constraint with name "idx" already exists
+ALTER TABLE t_name_check ADD CONSTRAINT idx PRIMARY KEY (a)
+
 # The following subtest tests the case when the new primary key
 # intersects with the old primary key.
 subtest regression_85877

pkg/sql/schemachanger/scbuild/internal/scbuildstmt/alter_table_alter_primary_key.go

@@ -133,6 +133,7 @@ func alterPrimaryKey(b BuildCtx, tn *tree.TableName, tbl *scpb.Table, t alterPri
 		}
 	}
 	out.apply(b.Drop)
+	checkIfConstraintNameAlreadyExists(b, tbl, t)
 	sharding := makeShardedDescriptor(b, t)
 	var sourcePrimaryIndexElem *scpb.PrimaryIndex
 	if rowidToDrop == nil {
@@ -499,6 +500,25 @@ func mustRetrieveKeyIndexColumns(
 	return indexColumns
 }
 
+func checkIfConstraintNameAlreadyExists(b BuildCtx, tbl *scpb.Table, t alterPrimaryKeySpec) {
+	if t.Name == "" {
+		return
+	}
+	// Check explicit constraint names.
+	publicTableElts := b.QueryByID(tbl.TableID).Filter(publicTargetFilter)
+	scpb.ForEachConstraintName(publicTableElts, func(_ scpb.Status, _ scpb.TargetStatus, e *scpb.ConstraintName) {
+		if e.Name == string(t.Name) {
+			panic(pgerror.Newf(pgcode.DuplicateObject, "constraint with name %q already exists", t.Name))
+		}
+	})
+	// Check index names.
+	scpb.ForEachIndexName(publicTableElts, func(_ scpb.Status, _ scpb.TargetStatus, n *scpb.IndexName) {
+		if n.Name == string(t.Name) {
+			panic(pgerror.Newf(pgcode.DuplicateObject, "constraint with name %q already exists", t.Name))
+		}
+	})
+}
+
 // makeShardedDescriptor construct a sharded descriptor for the new primary key.
 // Return nil if the new primary key is not hash-sharded.
 func makeShardedDescriptor(b BuildCtx, t alterPrimaryKeySpec) *catpb.ShardedDescriptor {