sql: avoid checking if a role exists when setting to the current role

This is an uncached round-trip. It makes `DISCARD` expensive.
In #86485 we implemented
`SET SESSION AUTHORIZATION DEFAULT`, this added an extra sql query to
`DISCARD ALL` to check if the role we'd become exists. This is almost
certainly unintentional in the case where the role we'd become is the
current session role. I think this just happened because of code
consolidation.

We now no longer check if the current session role exists. This removes
the last round-trip from DISCARD ALL.

Release note (performance improvement): In 22.2, logic was added to make
`SET SESSION AUTHORIZATION DEFAULT` not a no-op. This implementation used
more general code for setting the role for a session which made sure that
the role exists. The check for whether a role exists is currently uncached.
We don't need to check if the role we already are exists. This improves the
performance of `DISCARD ALL` in addition to `SET SESSION AUTHORIZATION
DEFAULT`.

pkg/bench/rttanalysis/testdata/benchmark_expectations

@@ -31,9 +31,9 @@ exp,benchmark
 15,CreateRole/create_role_with_2_options
 18,CreateRole/create_role_with_3_options
 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"
-1,"Discard/DISCARD_ALL,_no_tables"
+12,"Discard/DISCARD_ALL,_1_tables_in_1_db"
+15,"Discard/DISCARD_ALL,_2_tables_in_2_dbs"
+0,"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/user.go

@@ -546,7 +546,7 @@ func (p *planner) setRole(ctx context.Context, local bool, s username.SQLUsernam
 	sessionUser := p.SessionData().SessionUser()
 	becomeUser := sessionUser
 	// Check the role exists - if so, populate becomeUser.
-	if !s.IsNoneRole() {
+	if !s.IsNoneRole() && s != sessionUser {
 		becomeUser = s
 
 		exists, err := p.RoleExists(ctx, becomeUser)