[BHJ] fix side selection for left join with large right side (#43761)

2 files changed, 94 insertions, 2 deletions

diff --git a/ydb/core/kqp/opt/logical/kqp_opt_cbo.cpp b/ydb/core/kqp/opt/logical/kqp_opt_cbo.cpp
index 6b274d0a177..b4de5fdbcac 100644
--- a/ydb/core/kqp/opt/logical/kqp_opt_cbo.cpp
+++ b/ydb/core/kqp/opt/logical/kqp_opt_cbo.cpp
@@ -251,8 +251,8 @@ double TKqpProviderContext::ComputeJoinCost(
                 + CONSTS_GRACEJOIN_OUTPUT_MULT * std::pow(estimatedOutputByteSize, CONSTS_GRACEJOIN_OUTPUT_POW));
         }
         case EJoinAlgoType::ReverseBlockJoin: {
-            return 1.5 * (CONSTS_GRACEJOIN_LEFT_SIDE_MULT * std::pow(leftSideByteSize, CONSTS_GRACEJOIN_LEFT_SIDE_POW)
-                + CONSTS_GRACEJOIN_RIGHT_SIDE_MULT * std::pow(rightSideByteSize, CONSTS_GRACEJOIN_RIGHT_SIDE_POW)
+            return 1.5 * (CONSTS_GRACEJOIN_LEFT_SIDE_MULT * std::pow(rightSideByteSize, CONSTS_GRACEJOIN_LEFT_SIDE_POW)
+                + CONSTS_GRACEJOIN_RIGHT_SIDE_MULT * std::pow(leftSideByteSize, CONSTS_GRACEJOIN_RIGHT_SIDE_POW)
                 + CONSTS_GRACEJOIN_OUTPUT_MULT * std::pow(estimatedOutputByteSize, CONSTS_GRACEJOIN_OUTPUT_POW));
         }
         default:
diff --git a/ydb/core/kqp/ut/join/kqp_block_hash_join_ut.cpp b/ydb/core/kqp/ut/join/kqp_block_hash_join_ut.cpp
index f2efa760338..6335c9de5fb 100644
--- a/ydb/core/kqp/ut/join/kqp_block_hash_join_ut.cpp
+++ b/ydb/core/kqp/ut/join/kqp_block_hash_join_ut.cpp
@@ -332,6 +332,98 @@ Y_UNIT_TEST_SUITE(KqpBlockHashJoin) {
         }
     }
 
+    Y_UNIT_TEST(BlockHashJoinLeftJoinBuildLeftSide) {
+        TKikimrSettings settings = TKikimrSettings().SetWithSampleTables(false);
+        settings.AppConfig.MutableTableServiceConfig()->SetEnableOlapSink(true);
+        TKikimrRunner kikimr(settings);
+
+        auto queryClient = kikimr.GetQueryClient();
+        {
+            auto status = queryClient.ExecuteQuery(
+                R"(
+                    CREATE TABLE `/Root/left_table` (
+                        id Int32 NOT NULL,
+                        data String NOT NULL,
+                        PRIMARY KEY (id, data)
+                    )
+                    WITH (STORE = COLUMN);
+
+                    CREATE TABLE `/Root/right_table` (
+                        id Int32 NOT NULL,
+                        data String NOT NULL,
+                        PRIMARY KEY (id, data)
+                    )
+                    WITH (STORE = COLUMN);
+                )",  NYdb::NQuery::TTxControl::NoTx()
+            ).GetValueSync();
+            UNIT_ASSERT_C(status.IsSuccess(), status.GetIssues().ToString());
+        }
+
+        {
+            auto status = queryClient.ExecuteQuery(
+                R"(
+                    INSERT INTO `/Root/left_table` (id, data) VALUES
+                        (1, "1"),
+                        (2, "2"),
+                        (3, "3"),
+                        (4, "4");
+
+                    INSERT INTO `/Root/right_table` (id, data) VALUES
+                        (1, "1"),
+                        (2, "2"),
+                        (3, "3");
+                )", NYdb::NQuery::TTxControl::BeginTx().CommitTx()
+            ).GetValueSync();
+            UNIT_ASSERT_C(status.IsSuccess(), status.GetIssues().ToString());
+        }
+
+        {
+            TString hints = R"(
+                PRAGMA TablePathPrefix='/Root';
+                PRAGMA ydb.OptimizerHints=
+                    '
+                        Bytes(L # 10e9)
+                        Bytes(R # 10e20)
+                        Rows(L # 10e9)
+                        Rows(R # 10e20)
+                    ';
+            )";
+            TString blocks = "PRAGMA ydb.UseBlockHashJoin = \"true\";\n\n";
+            TString select = R"(
+                SELECT L.id AS left_id, L.data AS left_data, R.id AS right_id, R.data AS right_data
+                FROM `left_table` AS L
+                LEFT JOIN `right_table` AS R
+                ON L.id = R.id AND L.data = R.data
+                ORDER BY left_id;
+            )";
+
+            TString joinQuery = TStringBuilder() << hints << blocks << select;
+
+            auto status = queryClient.ExecuteQuery(joinQuery, NYdb::NQuery::TTxControl::BeginTx().CommitTx()).GetValueSync();
+            UNIT_ASSERT_C(status.IsSuccess(), status.GetIssues().ToString());
+
+            auto resultSet = status.GetResultSets()[0];
+            UNIT_ASSERT_VALUES_EQUAL(resultSet.RowsCount(), 4);
+
+            auto explainResult = queryClient.ExecuteQuery(
+                joinQuery,
+                NYdb::NQuery::TTxControl::NoTx(),
+                NYdb::NQuery::TExecuteQuerySettings().ExecMode(NYdb::NQuery::EExecMode::Explain)
+            ).GetValueSync();
+            UNIT_ASSERT_VALUES_EQUAL_C(explainResult.GetStatus(), EStatus::SUCCESS, explainResult.GetIssues().ToString());
+
+            auto astOpt = explainResult.GetStats()->GetAst();
+            UNIT_ASSERT(astOpt.has_value());
+            TString ast = TString(*astOpt);
+            Cout << "AST (LEFT JOIN, small left side -> build left): " << ast << Endl;
+
+            UNIT_ASSERT_C(ast.Contains("BlockHashJoin") || ast.Contains("DqBlockHashJoin"),
+                TStringBuilder() << "AST should contain BlockHashJoin. Actual AST: " << ast);
+            UNIT_ASSERT_C(ast.Contains(R"('('"BuildSide" '"Left")"),
+                TStringBuilder() << "AST should contain the BuildSide=Left. Actual AST: " << ast);
+        }
+    }
+
     Y_UNIT_TEST(BlockHashJoinLeftSemiJoin) {
         TKikimrSettings settings = TKikimrSettings().SetWithSampleTables(false);
         settings.AppConfig.MutableTableServiceConfig()->SetEnableOlapSink(true);