sql/schema: re-organize the UnleasableSystemDescriptors set

This commit re-organizes the static maps we use to perform lookups into the
`UnleasableSystemDescriptors`. It splits the `UnleasableSystemDescriptors` map
into two, one optimized for `leasedDescriptors.getByName` and one optimized for
`leasedDescriptors.getByID`.

In a 100% read workload, `leasedDescriptors` accesses (`getByName` and
`getByID`) were responsible for **4.39** of total CPU utilization.

Within this, about **0.33%** of total CPU utilization was spent directly in
these functions. This change shouldn't make a large difference (less than
**0.2%**), but it should improve things slightly. Since it also improves
readability, it seems worthwhile.

```
      File: cockroach
Type: cpu
Time: Dec 30, 2021 at 10:57pm (UTC)
Duration: 30.15s, Total samples = 74.99s (248.72%)
Active filters:
   focus=leasedDescriptors\).getBy
Showing nodes accounting for 3.29s, 4.39% of 74.99s total
----------------------------------------------------------+-------------
      flat  flat%   sum%        cum   cum%   calls calls% + context
----------------------------------------------------------+-------------
                                             0.14s   100% |   github.com/cockroachdb/cockroach/pkg/sql/catalog/descs.(*Collection).getByName /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descs/descriptor.go:199
     0.10s  0.13%  0.13%      0.14s  0.19%                | github.com/cockroachdb/cockroach/pkg/sql/catalog/descs.(*leasedDescriptors).getByName /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descs/leased_descriptors.go:102
                                             0.03s 21.43% |   github.com/cockroachdb/cockroach/pkg/sql/catalog/descpb.(*TableDescriptor).GetParentID /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descpb/structured.pb.go:2347
                                             0.01s  7.14% |   github.com/cockroachdb/cockroach/pkg/sql/catalog/dbdesc.(*immutable).GetParentID /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/dbdesc/database_desc.go:93
----------------------------------------------------------+-------------
                                             0.14s   100% |   github.com/cockroachdb/cockroach/pkg/sql/catalog/descs.(*Collection).getByName /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descs/descriptor.go:199
     0.04s 0.053%  0.19%      0.14s  0.19%                | github.com/cockroachdb/cockroach/pkg/sql/catalog/descs.(*leasedDescriptors).getByName /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descs/leased_descriptors.go:101
                                             0.03s 21.43% |   runtime.mapiternext /usr/local/go/src/runtime/map.go:851
                                             0.02s 14.29% |   runtime.mapiterinit /usr/local/go/src/runtime/map.go:821
                                             0.01s  7.14% |   runtime.duffzero /usr/local/go/src/runtime/duff_amd64.s:95
                                             0.01s  7.14% |   runtime.mapiterinit /usr/local/go/src/runtime/map.go:832
                                             0.01s  7.14% |   runtime.mapiterinit /usr/local/go/src/runtime/map.go:848
                                             0.01s  7.14% |   runtime.mapiternext /usr/local/go/src/runtime/map.go:898
                                             0.01s  7.14% |   runtime.mapiternext /usr/local/go/src/runtime/map.go:972
----------------------------------------------------------+-------------
```

pkg/sql/catalog/descs/leased_descriptors.go

@@ -98,12 +98,8 @@ func (ld *leasedDescriptors) getByName(
 		return cached.(lease.LeasedDescriptor).Underlying(), false, nil
 	}
 
-	for _, d := range systemschema.UnleasableSystemDescriptors {
-		if parentID == d.GetParentID() &&
-			parentSchemaID == d.GetParentSchemaID() &&
-			name == d.GetName() {
-			return nil, true, nil
-		}
+	if systemschema.IsUnleasableSystemDescriptorByName(parentID, parentSchemaID, name) {
+		return nil, true, nil
 	}
 
 	readTimestamp := txn.ReadTimestamp()
@@ -128,7 +124,7 @@ func (ld *leasedDescriptors) getByID(
 		return cached.(lease.LeasedDescriptor).Underlying(), false, nil
 	}
 
-	if _, isUnleasable := systemschema.UnleasableSystemDescriptors[id]; isUnleasable {
+	if systemschema.IsUnleasableSystemDescriptorByID(id) {
 		return nil, true, nil
 	}
 

pkg/sql/catalog/systemschema/system.go

@@ -2291,23 +2291,66 @@ var (
 			}}
 		},
 	)
+)
 
-	// UnleasableSystemDescriptors contains the system descriptors which cannot
-	// be leased. This includes the lease table itself, among others.
-	UnleasableSystemDescriptors = func(s []catalog.Descriptor) map[descpb.ID]catalog.Descriptor {
-		m := make(map[descpb.ID]catalog.Descriptor, len(s))
-		for _, d := range s {
-			m[d.GetID()] = d
-		}
-		return m
-	}([]catalog.Descriptor{
+type descRefByName struct {
+	parentID       descpb.ID
+	parentSchemaID descpb.ID
+	name           string
+}
+
+var (
+	// UnleasableSystemDescriptors contains the system descriptors which
+	// cannot be leased. This includes the lease table itself, among others.
+	UnleasableSystemDescriptors = []catalog.Descriptor{
 		SystemDB,
 		LeaseTable,
 		DescriptorTable,
 		NamespaceTable,
 		RangeEventTable,
-	})
+	}
+
+	unleasableSystemDescriptorsByID = func(s []catalog.Descriptor) map[descpb.ID]struct{} {
+		m := make(map[descpb.ID]struct{}, len(s))
+		for _, d := range s {
+			m[d.GetID()] = struct{}{}
+		}
+		return m
+	}(UnleasableSystemDescriptors)
+
+	unleasableSystemDescriptorsByName = func(s []catalog.Descriptor) map[descRefByName]struct{} {
+		m := make(map[descRefByName]struct{}, len(s))
+		for _, d := range s {
+			m[descRefByName{
+				parentID:       d.GetParentID(),
+				parentSchemaID: d.GetParentSchemaID(),
+				name:           d.GetName(),
+			}] = struct{}{}
+		}
+		return m
+	}(UnleasableSystemDescriptors)
 )
 
+// IsUnleasableSystemDescriptorByID returns whether the specified descriptor is
+// a member of the UnleasableSystemDescriptors set, given an ID.
+func IsUnleasableSystemDescriptorByID(id descpb.ID) bool {
+	_, ok := unleasableSystemDescriptorsByID[id]
+	return ok
+}
+
+// IsUnleasableSystemDescriptorByName returns whether the specified descriptor
+// is a member of the UnleasableSystemDescriptors set, given a database, schema,
+// and name.
+func IsUnleasableSystemDescriptorByName(
+	parentID descpb.ID, parentSchemaID descpb.ID, name string,
+) bool {
+	_, ok := unleasableSystemDescriptorsByName[descRefByName{
+		parentID:       parentID,
+		parentSchemaID: parentSchemaID,
+		name:           name,
+	}]
+	return ok
+}
+
 // SpanConfigurationsTableName represents system.span_configurations.
 var SpanConfigurationsTableName = tree.NewTableNameWithSchema("system", tree.PublicSchemaName, tree.Name(catconstants.SpanConfigurationsTableName))