sql: don't disable rewrites for internal SQL

Casting an OID to REGNAMESPACE runs this SQL using the internal executor:
```
SELECT pg_namespace.oid, nspname FROM pg_catalog.pg_namespace WHERE oid = $1
```
When the `GenerateConstrainedScans` rule is disabled, this returns no
rows and so doesn't apply the cast. The same SQL, not run via an
internal executor, but also with `GenerateConstrainedScans` disabled,
behaves correctly. Disabling `GenerateConstrainedScans` all the time
prevents the cluster from starting.

Internal SQL is sensitive to disabled optimizer rewrite rules, and it
could cause server instability to allow rules to ever be disabled for
internal SQL. Also, the `testing_optimizer_disable_rule_probability` session
setting is mainly meant to perturb query plans of foreground SQL in
randomized tests, and enabling it for internal SQL could make these
tests more noisy and create harder to debug issues.

Therefore the fix is to set the `testing_optimizer_disable_rule_probability`
setting to zero for internal SQL statements.

Fixes cockroachdb#98322

Release note: None

pkg/sql/internal.go

@@ -716,6 +716,12 @@ func applyInternalExecutorSessionExceptions(sd *sessiondata.SessionData) {
 	// DisableBuffering is not supported by the InternalExecutor
 	// which uses streamingCommandResults.
 	sd.LocalOnlySessionData.AvoidBuffering = false
+	// Rewrite rules should not be disabled for internal SQL. This is mainly a
+	// flag for testing foreground SQL. Disrupting the behavior of internal SQL,
+	// which may rely on rewrite rules for proper behavior, could potentially make
+	// the cluster unstable and may make randomized tests more noisy with
+	// difficult to debug behavior.
+	sd.LocalOnlySessionData.TestingOptimizerDisableRuleProbability = 0
 }
 
 // applyOverrides overrides the respective fields from sd for all the fields set on o.

pkg/sql/logictest/testdata/logic_test/namespace

@@ -108,3 +108,16 @@ ALTER INDEX a_pk3 RENAME TO a_pk4
 # But qualification resolves the problem.
 statement ok
 ALTER INDEX public.a_pk3 RENAME TO a_pk4
+
+statement ok
+SET testing_optimizer_disable_rule_probability=1.0
+
+# A cast which relies on an internal executor should not flake with rewrite
+# rules disabled.
+query T
+select regnamespace(101)::REGNAMESPACE;
+----
+public
+
+statement ok
+RESET testing_optimizer_disable_rule_probability