sql: disable buffered writes on the txn when it's used by IE

This commit is a follow-up to 07d8902
where we disabled the session variable that controls whether buffered
writes are enabled when the variable is consulted by the internal
executor. That change had an oversight - that if the IE is using already
created txn, the session variable has no influence, so this commit
explicitly disables buffered writes on the txn whenever it's used by the
IE for the first time.

Release note: None

Author: yuzefovich

pkg/sql/internal.go

@@ -1185,6 +1185,12 @@ func (ie *InternalExecutor) execInternal(
 
 	applyInternalExecutorSessionExceptions(sd)
 	applyOverrides(sessionDataOverride, sd)
+	if txn != nil && txn.Type() == kv.RootTxn {
+		// For 25.2, we're being conservative and explicitly disabling buffered
+		// writes for the internal executor.
+		// TODO(yuzefovich): remove this for 25.3.
+		txn.SetBufferedWritesEnabled(false)
+	}
 	attributeToUser := sessionDataOverride.AttributeToUser && attributeToUserEnabled.Get(&ie.s.cfg.Settings.SV)
 	growStackSize := sessionDataOverride.GrowStackSize
 	if !rw.async() && (txn != nil && txn.Type() == kv.RootTxn) {