sql: add pg_encoding_max_length builtin

This commit adds an implementation for the
`pg_encoding_max_length` builtin.

Needed for: cockroachdb#109603
Epic: none

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

pkg/sql/logictest/testdata/logic_test/pg_builtins

@@ -952,4 +952,38 @@ attname  typname   information_schema._pg_interval_type
 ubar  interval     NULL
 ufoo  interval     NULL
 
+statement ok
+DROP TYPE u;
+
 subtest end
+
+subtest information_schema._pg_char_octet_length
+
+query I
+SELECT information_schema._pg_char_octet_length(25, NULL)
+----
+NULL
+
+query I
+SELECT information_schema._pg_char_octet_length(25, -1)
+----
+1073741824
+
+statement ok
+CREATE TYPE u AS (ufoo char, ubar int);
+
+query TTI colnames
+SELECT a.attname,
+       t.typname,
+       information_schema._pg_char_octet_length(a.atttypid,a.atttypmod)
+FROM pg_attribute a
+JOIN pg_type t
+ON a.atttypid = t.oid
+WHERE a.attname = 'ufoo'
+ORDER BY a.attname
+----
+attname  typname  information_schema._pg_char_octet_length
+ufoo     bpchar   4
+
+subtest end
+

pkg/sql/sem/builtins/fixed_oids.go

@@ -2489,6 +2489,7 @@ var builtinOidsArray = []string{
 	2518: `information_schema._pg_char_octet_length(typid: oid, typmod: int4) -> int`,
 	2519: `information_schema._pg_datetime_precision(typid: oid, typmod: int4) -> int`,
 	2520: `information_schema._pg_interval_type(typid: oid, typmod: int4) -> string`,
+	2521: `pg_encoding_max_length(encoding: int) -> int`,
 }
 
 var builtinOidsBySignature map[string]oid.Oid

pkg/sql/sem/builtins/pg_builtins.go

@@ -1370,6 +1370,27 @@ var pgBuiltins = map[string]builtinDefinition{
 	// Note that this function was removed from Postgres in version 10.
 	"pg_is_xlog_replay_paused": makeNotUsableFalseBuiltin(),
 
+	// pg_encoding_max_length returns the maximum length of a given encoding. For CRDB's use case,
+	// we only support UTF8; so, this will return the max_length of UTF8 - which is 4.
+	// https://github.com/postgres/postgres/blob/master/src/common/wchar.c
+	"pg_encoding_max_length": makeBuiltin(
+		tree.FunctionProperties{},
+		tree.Overload{
+			Types:      tree.ParamTypes{{Name: "encoding", Typ: types.Int}},
+			ReturnType: tree.FixedReturnType(types.Int),
+			Fn: func(ctx context.Context, evalCtx *eval.Context, args tree.Datums) (tree.Datum, error) {
+				if cmp, err := args[0].CompareError(evalCtx, DatEncodingUTFId); err != nil {
+					return tree.DNull, err
+				} else if cmp == 0 {
+					return tree.NewDInt(4), nil
+				}
+				return tree.DNull, nil
+			},
+			Info:       notUsableInfo,
+			Volatility: volatility.Immutable,
+		},
+	),
+
 	// Access Privilege Inquiry Functions allow users to query object access
 	// privileges programmatically. Each function has a number of variants,
 	// which differ based on their function signatures. These signatures have