sql: add information_schema._pg_char_octet_length builtin

This commit adds an implementation for the
`information_schema._pg_char_octet_length` 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_char_octet_length`
builtin function is now supported, which improves compatibility with
PostgreSQL.

pkg/sql/sem/builtins/fixed_oids.go

@@ -2486,6 +2486,7 @@ var builtinOidsArray = []string{
 	2515: `crdb_internal.privilege_name(internal_key: string) -> string`,
 	2516: `crdb_internal.privilege_name(internal_key: string[]) -> string[]`,
 	2517: `jsonb_array_to_string_array(input: jsonb) -> string[]`,
+	2518: `information_schema._pg_char_octet_length(typid: oid, typmod: int4) -> int`,
 }
 
 var builtinOidsBySignature map[string]oid.Oid

pkg/sql/sem/builtins/pg_builtins.go

@@ -2115,7 +2115,7 @@ var pgBuiltins = map[string]builtinDefinition{
 					return tree.NewDInt(64), nil
 				case oid.T_numeric:
 					if typmod != -1 {
-						// This logics matches the postgres implementation
+						// This logic matches the postgres implementation
 						// of how to calculate the precision based on the typmod
 						// https://github.com/postgres/postgres/blob/d84ffffe582b8e036a14c6bc2378df29167f3a00/src/backend/catalog/information_schema.sql#L109
 						return tree.NewDInt(((typmod - 4) >> 16) & 65535), nil
@@ -2183,6 +2183,30 @@ var pgBuiltins = map[string]builtinDefinition{
 		},
 	),
 
+	// https://github.com/postgres/postgres/blob/master/src/backend/catalog/information_schema.sql
+	"information_schema._pg_char_octet_length": 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),
+			Body: `SELECT
+						 CASE WHEN $1 IN (25, 1042, 1043) /* text, char, varchar */
+			            THEN CASE WHEN $2 = -1 /* default typmod */
+														THEN CAST(2^30 AS integer)
+			                    	ELSE information_schema._pg_char_max_length($1, $2) *
+			                           pg_catalog.pg_encoding_max_length((SELECT encoding FROM pg_catalog.pg_database WHERE datname = pg_catalog.current_database()))
+			                 END
+			            ELSE null
+			       END`,
+			Info:              notUsableInfo,
+			Volatility:        volatility.Immutable,
+			CalledOnNullInput: true,
+			Language:          tree.RoutineLangSQL,
+		},
+	),
+
 	"nameconcatoid": makeBuiltin(
 		tree.FunctionProperties{
 			Category: builtinconstants.CategorySystemInfo,