sql: add information_schema._pg_datetime_precision builtin

This commit adds an implementation for the
`information_schema._pg_datetime_precision` builtin.

The builtin is implemented as a user-defined function in Postgres
[here](https://github.com/postgres/postgres/blob/master/src/backend/catalog/information_schema.sql)

Needed for: cockroachdb#109603
Epic: none

Release note (sql change): The
`information_schema._pg_datetime_precision` builtin function is
now supported, which improves compatibility with PostgreSQL.

pkg/sql/logictest/testdata/logic_test/pg_builtins

@@ -892,14 +892,14 @@ subtest end
 subtest information_schema._pg_char_octet_length
 
 query I
-SELECT information_schema._pg_char_octet_length(25, NULL)
+SELECT information_schema._pg_char_octet_length(25, -1)
 ----
-NULL
+1073741824
 
 query I
-SELECT information_schema._pg_char_octet_length(25, -1)
+SELECT information_schema._pg_char_octet_length(25, NULL)
 ----
-1073741824
+NULL
 
 statement ok
 CREATE TYPE u AS (ufoo char, ubar int);
@@ -935,3 +935,42 @@ SELECT pg_encoding_max_length(1)
 NULL
 
 subtest end
+
+subtest information_schema._pg_datetime_precision
+
+query I
+select information_schema._pg_datetime_precision(1082, -1);
+----
+0
+
+query I
+select information_schema._pg_datetime_precision(1083, -1);
+----
+6
+
+query I
+select information_schema._pg_datetime_precision(1083, 5);
+----
+5
+
+query I
+select information_schema._pg_datetime_precision(1186, -1);
+----
+6
+
+query I
+select information_schema._pg_datetime_precision(1186, 5);
+----
+5
+
+query I
+select information_schema._pg_datetime_precision(1086, 5);
+----
+NULL
+
+query I
+select information_schema._pg_datetime_precision(1186, NULL);
+----
+NULL
+
+subtest end

pkg/sql/sem/builtins/fixed_oids.go

@@ -2508,6 +2508,7 @@ var builtinOidsArray = []string{
 	2537: `percentile_disc_impl(arg1: float[], arg2: refcursor) -> refcursor[]`,
 	2538: `information_schema._pg_char_octet_length(typid: oid, typmod: int4) -> int`,
 	2539: `pg_encoding_max_length(encoding: int) -> int`,
+	2540: `information_schema._pg_datetime_precision(typid: oid, typmod: int4) -> int`,
 }
 
 var builtinOidsBySignature map[string]oid.Oid

pkg/sql/sem/builtins/pg_builtins.go

@@ -2228,6 +2228,53 @@ var pgBuiltins = map[string]builtinDefinition{
 		},
 	),
 
+	// NOTE: this could be defined as a user-defined function, like
+	// it is in Postgres:
+	// https://github.com/postgres/postgres/blob/master/src/backend/catalog/information_schema.sql
+	// CREATE FUNCTION _pg_datetime_precision(typid oid, typmod int4) RETURNS integer
+	//     LANGUAGE sql
+	//     IMMUTABLE
+	//     PARALLEL SAFE
+	//     RETURNS NULL ON NULL INPUT
+	// RETURN
+	//   CASE WHEN $1 IN (1082) /* date */
+	// 						THEN 0
+	// 	 			WHEN $1 IN (1083, 1114, 1184, 1266) /* time, timestamp, same + tz */
+	// 						THEN CASE WHEN $2 < 0 THEN 6 ELSE $2 END
+	// 				WHEN $1 IN (1186) /* interval */
+	// 						THEN CASE WHEN $2 < 0 OR $2 & 0xFFFF = 0xFFFF THEN 6 ELSE $2 & 0xFFFF END
+	// 				ELSE null
+	// END;
+	"information_schema._pg_datetime_precision": makeBuiltin(tree.FunctionProperties{Category: builtinconstants.CategorySystemInfo},
+		tree.Overload{
+			Types: tree.ParamTypes{
+				{Name: "typid", Typ: types.Oid},
+				{Name: "typmod", Typ: types.Int4},
+			},
+			ReturnType: tree.FixedReturnType(types.Int),
+			Fn: func(_ context.Context, _ *eval.Context, args tree.Datums) (tree.Datum, error) {
+				typid := args[0].(*tree.DOid).Oid
+				typmod := *args[1].(*tree.DInt)
+				if typid == oid.T_date {
+					return tree.DZero, nil
+				} else if typid == oid.T_time || typid == oid.T_timestamp || typid == oid.T_timestamptz || typid == oid.T_timetz {
+					if typmod < 0 {
+						return tree.NewDInt(6), nil
+					}
+					return tree.NewDInt(typmod), nil
+				} else if typid == oid.T_interval {
+					if typmod < 0 || (typmod&0xFFFF) == 0xFFFF {
+						return tree.NewDInt(6), nil
+					}
+					return tree.NewDInt(typmod & 0xFFFF), nil
+				}
+				return tree.DNull, nil
+			},
+			Info:       notUsableInfo,
+			Volatility: volatility.Immutable,
+		},
+	),
+
 	"nameconcatoid": makeBuiltin(
 		tree.FunctionProperties{
 			Category: builtinconstants.CategorySystemInfo,