introduce snapshot_ro isolation level (ydb-platform#11085)

Author: ssmike

ydb/core/kqp/common/kqp_tx.cpp

@@ -157,7 +157,8 @@ bool NeedSnapshot(const TKqpTransactionContext& txCtx, const NYql::TKikimrConfig
 {
     Y_UNUSED(config);
 
-    if (*txCtx.EffectiveIsolationLevel != NKikimrKqp::ISOLATION_LEVEL_SERIALIZABLE)
+    if (*txCtx.EffectiveIsolationLevel != NKikimrKqp::ISOLATION_LEVEL_SERIALIZABLE &&
+        *txCtx.EffectiveIsolationLevel != NKikimrKqp::ISOLATION_LEVEL_SNAPSHOT_RO)
         return false;
 
     if (txCtx.GetSnapshot().IsValid())

ydb/core/kqp/common/kqp_tx.h

@@ -256,8 +256,7 @@ class TKqpTransactionContext : public NYql::TKikimrTransactionContextBase  {
                 break;
 
             case Ydb::Table::TransactionSettings::kSnapshotReadOnly:
-                // TODO: (KIKIMR-3374) Use separate isolation mode to avoid optimistic locks.
-                EffectiveIsolationLevel = NKikimrKqp::ISOLATION_LEVEL_SERIALIZABLE;
+                EffectiveIsolationLevel = NKikimrKqp::ISOLATION_LEVEL_SNAPSHOT_RO;
                 Readonly = true;
                 break;
 

ydb/core/kqp/executer_actor/kqp_planner.cpp

@@ -187,8 +187,8 @@ std::unique_ptr<TEvKqpNode::TEvStartKqpTasksRequest> TKqpPlanner::SerializeReque
     request.SetTxId(TxId);
     if (LockTxId) {
         request.SetLockTxId(*LockTxId);
+        request.SetLockNodeId(LockNodeId);
     }
-    request.SetLockNodeId(LockNodeId);
     ActorIdToProto(ExecuterId, request.MutableExecuterActorId());
 
     if (Deadline) {

ydb/core/protos/kqp.proto

@@ -64,6 +64,7 @@ enum EIsolationLevel {
     ISOLATION_LEVEL_READ_COMMITTED = 2;
     ISOLATION_LEVEL_READ_UNCOMMITTED = 3;
     ISOLATION_LEVEL_READ_STALE = 4;
+    ISOLATION_LEVEL_SNAPSHOT_RO = 5;
 };
 
 enum EQueryReplyFlags {