sql: disallow partial unique constraints in ALTER TABLE

Previously, a `WHERE` clause in `ALTER TABLE .. ADD CONSTRAINT .. UNIQUE`
statements was parsed successfully but ignored so that the resulting
unique constraint was not partial. This commit disallows the `WHERE`
clause. Postgres allows partial unique indexes, but not partial unique
constraints, so we will likely remove support for partial unique
constraints in `CREATE TABLE` statements in an upcoming major release.
See discussion in cockroachdb#65825.

Fixes cockroachdb#67234

Release note (bug fix/sql change): A bug was identified that created
non-partial unique constraints when a user attempted to create a partial
unique constraint in `ALTER TABLE` statements. Creating partial unique
constraints in `ALTER TABLE` is now disallowed. A partial unique index
can be created instead with `CREATE UNIQUE INDEX .. WHERE`.

pkg/sql/alter_table.go

@@ -201,6 +201,12 @@ func (n *alterTableNode) startExec(params runParams) error {
 					continue
 				}
 
+				if d.Predicate != nil {
+					return pgerror.New(pgcode.InvalidTableDefinition,
+						"partial unique constraints cannot be added in ALTER TABLE, "+
+							"use CREATE UNIQUE INDEX instead")
+				}
+
 				if d.PrimaryKey {
 					// We only support "adding" a primary key when we are using the
 					// default rowid primary index or if a DROP PRIMARY KEY statement

pkg/sql/logictest/testdata/logic_test/alter_table

@@ -1481,7 +1481,12 @@ ALTER TABLE unique_without_index ADD COLUMN f INT UNIQUE WITHOUT INDEX
 # constraint.
 statement ok
 ALTER TABLE unique_without_index ADD COLUMN f INT;
-ALTER TABLE unique_without_index ADD CONSTRAINT my_unique_f UNIQUE WITHOUT INDEX (f);
+ALTER TABLE unique_without_index ADD CONSTRAINT my_unique_f UNIQUE WITHOUT INDEX (f)
+
+# Partial predicates are allowed for UNIQUE WITHOUT INDEX, but not UNIQUE.
+# TODO(mgartner): This is confusing, and something we should think more about
+# if we ever make UNIQUE WITHOUT INDEX available by default.
+statement ok
 ALTER TABLE unique_without_index ADD CONSTRAINT my_partial_unique_f UNIQUE WITHOUT INDEX (f) WHERE f > 0
 
 # The unique constraint predicate must be valid. It cannot reference

pkg/sql/logictest/testdata/logic_test/partial_index

@@ -76,6 +76,14 @@ CREATE TABLE t4 (a INT, UNIQUE INDEX (a) WHERE a = 0)
 statement error expected index predicate expression to have type bool, but '1' has type int
 CREATE TABLE error (a INT, UNIQUE INDEX (a) WHERE 1)
 
+# Disallow partial unique constraints in ALTER TABLE.
+
+statement ok
+CREATE TABLE up (a INT)
+
+statement error partial unique constraints cannot be added in ALTER TABLE, use CREATE UNIQUE INDEX instead
+ALTER TABLE up ADD CONSTRAINT partial_unique UNIQUE (a) WHERE a > 0
+
 # Validate CREATE INDEX predicate.
 
 statement ok

pkg/sql/parser/sql.y

@@ -6801,6 +6801,9 @@ constraint_elem:
       Expr: $3.expr(),
     }
   }
+// TODO(mgartner): Postgres does not allow partial unique constraints. It only
+// allows partial unique indexes. The opt_where_clause should be removed so that
+// it is a syntax error. See discussion in #65825.
 | UNIQUE opt_without_index '(' index_params ')'
     opt_storing opt_interleave opt_partition_by_index opt_deferrable opt_where_clause
   {