colbuilder: don't use direct scans when buffered writes are enabled

`txnWriteBuffer` currently doesn't support COL_BATCH_RESPONSE scan
format, so if buffered writes are enabled, we won't use the direct scans.

We could've relaxed this condition further to not use direct scans if
at least some writes are actually buffered, but given this feature is
in experimental state, we don't bother doing so for now.

Release note: None

Author: yuzefovich

pkg/sql/colexec/colbuilder/execplan.go

@@ -842,6 +842,17 @@ func NewColOperator(
 			var resultTypes []*types.T
 			if flowCtx.EvalCtx.SessionData().DirectColumnarScansEnabled {
 				canUseDirectScan := func() bool {
+					// txnWriteBuffer currently doesn't support
+					// COL_BATCH_RESPONSE scan format, so if buffered writes are
+					// enabled, we won't use the direct scans.
+					//
+					// We could've relaxed this condition further to not use
+					// direct scans if at least some writes are actually
+					// buffered, but given this feature is in experimental
+					// state, we don't bother doing so for now.
+					if flowCtx.Txn.BufferedWritesEnabled() {
+						return false
+					}
 					// We currently don't use the direct scans if TraceKV is
 					// enabled (due to not being able to tell the KV server
 					// about it). One idea would be to include this boolean into