sql/sem/tree: make function names properly case-sensitive

There was a time in the past where CockroachDB was (or attempted to
be) case-insensitive about identifiers, and this was a source of
incompatibility with PostgreSQL.

This incompatibility was corrected for object
(table/view/database/index) names and column names in #16884, but
somehow I forgot about function names in that patch. As a result,
CockroachDB continued to consider `upper()` and `"UPPER"()` the same
thing, whereas most definitely only the former is valid.

This patch corrects the issue and simplifies the code accordingly.

Release note (sql change): CockroachDB now properly rejects
incorrectly cased SQL function names with an error.

pkg/sql/logictest/testdata/logic_test/function_lookup

@@ -33,3 +33,8 @@ query T
 INSERT INTO foo(x) VALUES (42) RETURNING PG_TYPEOF(x)
 ----
 int
+
+# CockroachDB is case-preserving for quoted identifiers like pg, and
+# function names only exist in lowercase.
+query error unknown function: PG_TYPEOF()
+SELECT "PG_TYPEOF"(123)

pkg/sql/sem/builtins/all_builtins.go

@@ -16,7 +16,6 @@ package builtins
 
 import (
 	"sort"
-	"strings"
 
 	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
 	"github.com/cockroachdb/cockroach/pkg/sql/sem/types"
@@ -40,18 +39,14 @@ func init() {
 		AllBuiltinNames = append(AllBuiltinNames, name)
 	}
 
-	// We alias the builtins to uppercase to hasten the lookup in the
-	// common case. Also generate missing categories.
+	// Generate missing categories.
 	for _, name := range AllBuiltinNames {
-		uname := strings.ToUpper(name)
 		def := Builtins[name]
 		for i, b := range def {
 			if b.Category == "" {
 				def[i].Category = getCategory(def[i])
 			}
 		}
-		Builtins[uname] = def
-		tree.FunDefs[uname] = tree.FunDefs[name]
 	}
 
 	sort.Strings(AllBuiltinNames)

pkg/sql/sem/tree/function_definition.go

@@ -15,8 +15,6 @@
 package tree
 
 import (
-	"strings"
-
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgerror"
 	"github.com/cockroachdb/cockroach/pkg/sql/sessiondata"
 )
@@ -80,25 +78,16 @@ func (n *UnresolvedName) ResolveFunction(
 		return d, nil
 	}
 
-	// Although the conversion from Name to string should go via
-	// Name.Normalize(), functions are special in that they are
-	// guaranteed to not contain special Unicode characters. So we can
-	// use ToLower directly.
-	// TODO(knz): this will need to be revisited once we allow
-	// function names to exist in custom namespaces, whose names
-	// may contain special characters.
-	prefix = strings.ToLower(prefix)
-	smallName := strings.ToLower(function)
-	fullName := smallName
+	fullName := function
 
-	if prefix == "pg_catalog" {
+	if prefix == sessiondata.PgCatalogName {
 		// If the user specified e.g. `pg_catalog.max()` we want to find
 		// it in the global namespace.
 		prefix = ""
 	}
 
 	if prefix != "" {
-		fullName = prefix + "." + smallName
+		fullName = prefix + "." + function
 	}
 	def, ok := FunDefs[fullName]
 	if !ok {
@@ -108,7 +97,7 @@ func (n *UnresolvedName) ResolveFunction(
 			// the search path first.
 			iter := searchPath.Iter()
 			for alt, ok := iter(); ok; alt, ok = iter() {
-				fullName = alt + "." + smallName
+				fullName = alt + "." + function
 				if def, ok = FunDefs[fullName]; ok {
 					found = true
 					break