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
 14,CreateRole/create_role_with_no_options
 19,"Discard/DISCARD_ALL,_1_tables_in_1_db"
 47,"Discard/DISCARD_ALL,_2_tables_in_2_dbs"
-9,"Discard/DISCARD_ALL,_no_tables"
+1,"Discard/DISCARD_ALL,_no_tables"
 14,DropDatabase/drop_database_0_tables
 16,DropDatabase/drop_database_1_table
 18,DropDatabase/drop_database_2_tables

pkg/sql/discard.go

@@ -86,6 +86,11 @@ 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
+	}
 	return p.WithInternalExecutor(ctx, func(ctx context.Context, txn *kv.Txn, ie sqlutil.InternalExecutor) error {
 		codec := p.execCfg.Codec
 		descCol := p.Descriptors()