sql: use in-memory session data to decide what to do in DISCARD

In #86246 we introduced logic to discard schemas when running DISCARD ALL and
DISCARD TEMP. This logic did expensive kv operations unconditionally; if the
session knew it had never created a temporary schema, we'd still fetch all
databases and proceed to search all databases for a temp schema. This is very
expensive.

Fixes: #95864

Release note (performance improvement): In 22.2, we introduced support for
`DISCARD TEMP` and made `DISCARD ALL` actually discard temp tables. This
implementation ran expensive logic to discover temp schemas rather than
consulting in-memory data structures. As a result, `DISCARD ALL`, which
is issued regularly by connection pools, became an expensive operation
when it should be cheap. This problem is now resolved.

pkg/bench/rttanalysis/testdata/benchmark_expectations

@@ -33,7 +33,7 @@ exp,benchmark
 13,CreateRole/create_role_with_no_options
 18,"Discard/DISCARD_ALL,_1_tables_in_1_db"
 21,"Discard/DISCARD_ALL,_2_tables_in_2_dbs"
-9,"Discard/DISCARD_ALL,_no_tables"
+1,"Discard/DISCARD_ALL,_no_tables"
 10,DropDatabase/drop_database_0_tables
 11,DropDatabase/drop_database_1_table
 11,DropDatabase/drop_database_2_tables

pkg/sql/discard.go

@@ -84,14 +84,26 @@ func (n *discardNode) startExec(params runParams) error {
 }
 
 func deleteTempTables(ctx context.Context, p *planner) error {
+	// If this session has no temp schemas, then there is nothing to do here.
+	// This is the common case.
+	if len(p.SessionData().DatabaseIDToTempSchemaID) == 0 {
+		return nil
+	}
 	codec := p.execCfg.Codec
 	descCol := p.Descriptors()
+	// Note: grabbing all the databases here is somewhat suspect. It appears
+	// that the logic related to maintaining the set of database temp schemas
+	// is somewhat incomplete, so there can be temp schemas in the sessiondata
+	// map which don't exist any longer.
 	allDbDescs, err := descCol.GetAllDatabaseDescriptors(ctx, p.Txn())
 	if err != nil {
 		return err
 	}
 	g := p.byNameGetterBuilder().MaybeGet()
 	for _, db := range allDbDescs {
+		if _, ok := p.SessionData().DatabaseIDToTempSchemaID[uint32(db.GetID())]; !ok {
+			continue
+		}
 		sc, err := g.Schema(ctx, db, p.TemporarySchemaName())
 		if err != nil {
 			return err