sql/catalog/descs: optimize access to the system db

We tend to access the system database a lot and we never cache it in the
lease manager. Before this patch, we'd always copy and re-allocate a pair
of them. There's no need to do that.

```
name                                           old time/op    new time/op    delta
FlowSetup/vectorize=true/distribute=true-16       141µs ± 3%     132µs ± 4%  -6.14%  (p=0.000 n=19+20)
FlowSetup/vectorize=true/distribute=false-16      138µs ± 4%     128µs ± 3%  -7.36%  (p=0.000 n=19+19)
FlowSetup/vectorize=false/distribute=true-16      134µs ± 2%     123µs ± 3%  -7.78%  (p=0.000 n=20+20)
FlowSetup/vectorize=false/distribute=false-16     129µs ± 3%     119µs ± 3%  -7.51%  (p=0.000 n=20+19)

name                                           old alloc/op   new alloc/op   delta
FlowSetup/vectorize=true/distribute=true-16      38.1kB ± 2%    36.5kB ± 2%  -4.19%  (p=0.000 n=18+18)
FlowSetup/vectorize=true/distribute=false-16     36.2kB ± 0%    34.6kB ± 0%  -4.32%  (p=0.000 n=17+17)
FlowSetup/vectorize=false/distribute=true-16     42.6kB ± 0%    41.1kB ± 0%  -3.63%  (p=0.000 n=18+16)
FlowSetup/vectorize=false/distribute=false-16    41.0kB ± 0%    39.4kB ± 0%  -3.76%  (p=0.000 n=16+18)

name                                           old allocs/op  new allocs/op  delta
FlowSetup/vectorize=true/distribute=true-16         368 ± 0%       342 ± 0%  -7.07%  (p=0.000 n=16+17)
FlowSetup/vectorize=true/distribute=false-16        354 ± 0%       328 ± 0%  -7.34%  (p=0.000 n=18+17)
FlowSetup/vectorize=false/distribute=true-16        337 ± 0%       312 ± 1%  -7.63%  (p=0.000 n=19+19)
FlowSetup/vectorize=false/distribute=false-16       325 ± 0%       299 ± 0%  -8.00%  (p=0.000 n=17+18)
```

Release note: None

pkg/sql/catalog/descs/uncommitted_descriptors.go

@@ -12,9 +12,11 @@ package descs
 
 import (
 	"github.com/cockroachdb/cockroach/pkg/sql/catalog"
+	"github.com/cockroachdb/cockroach/pkg/sql/catalog/dbdesc"
 	"github.com/cockroachdb/cockroach/pkg/sql/catalog/descpb"
 	"github.com/cockroachdb/cockroach/pkg/sql/catalog/lease"
 	"github.com/cockroachdb/cockroach/pkg/sql/catalog/nstree"
+	"github.com/cockroachdb/cockroach/pkg/sql/catalog/systemschema"
 	"github.com/cockroachdb/cockroach/pkg/sql/catalog/typedesc"
 	"github.com/cockroachdb/cockroach/pkg/util/iterutil"
 	"github.com/cockroachdb/errors"
@@ -113,6 +115,7 @@ func (ud *uncommittedDescriptors) add(mut catalog.MutableDescriptor) (catalog.De
 // checkOut checks out an uncommitted mutable descriptor for use in the
 // transaction. This descriptor should later be checked in again.
 func (ud *uncommittedDescriptors) checkOut(id descpb.ID) (catalog.MutableDescriptor, error) {
+	ud.maybeInitialize()
 	entry := ud.descs.GetByID(id)
 	if entry == nil {
 		return nil, errors.NewAssertionErrorWithWrappedErrf(
@@ -170,6 +173,7 @@ func maybeRefreshCachedFieldsOnTypeDescriptor(
 
 // getByID looks up an uncommitted descriptor by ID.
 func (ud *uncommittedDescriptors) getByID(id descpb.ID) catalog.Descriptor {
+	ud.maybeInitialize()
 	entry := ud.descs.GetByID(id)
 	if entry == nil {
 		return nil
@@ -187,6 +191,7 @@ func (ud *uncommittedDescriptors) getByID(id descpb.ID) catalog.Descriptor {
 func (ud *uncommittedDescriptors) getByName(
 	dbID descpb.ID, schemaID descpb.ID, name string,
 ) (hasKnownRename bool, desc catalog.Descriptor) {
+	ud.maybeInitialize()
 	// Walk latest to earliest so that a DROP followed by a CREATE with the same
 	// name will result in the CREATE being seen.
 	if got := ud.descs.GetByName(dbID, schemaID, name); got != nil {
@@ -202,6 +207,7 @@ func (ud *uncommittedDescriptors) getByName(
 func (ud *uncommittedDescriptors) iterateNewVersionByID(
 	fn func(entry catalog.NameEntry, originalVersion lease.IDVersion) error,
 ) error {
+	ud.maybeInitialize()
 	return ud.descs.IterateByID(func(entry catalog.NameEntry) error {
 		mut := entry.(*uncommittedDescriptor).mutable
 		if mut.IsNew() || !mut.IsUncommittedVersion() {
@@ -214,6 +220,7 @@ func (ud *uncommittedDescriptors) iterateNewVersionByID(
 func (ud *uncommittedDescriptors) iterateImmutableByID(
 	fn func(imm catalog.Descriptor) error,
 ) error {
+	ud.maybeInitialize()
 	return ud.descs.IterateByID(func(entry catalog.NameEntry) error {
 		return fn(entry.(*uncommittedDescriptor).immutable)
 	})
@@ -262,3 +269,17 @@ func (ud *uncommittedDescriptors) hasUncommittedTypes() (has bool) {
 	})
 	return has
 }
+
+var systemUncommittedDatabase = func() *uncommittedDescriptor {
+	ud, err := makeUncommittedDescriptor(dbdesc.NewBuilder(systemschema.SystemDB.DatabaseDesc()).BuildExistingMutable())
+	if err != nil {
+		panic(err)
+	}
+	return ud
+}()
+
+func (ud *uncommittedDescriptors) maybeInitialize() {
+	if ud.descs.Len() == 0 {
+		ud.descs.Upsert(systemUncommittedDatabase)
+	}
+}