row dispatcher/format_handler: restore compatibility (#27705)

Author: yumkam

ydb/core/fq/libs/row_dispatcher/format_handler/filters/purecalc_filter.cpp

@@ -432,14 +432,19 @@ class TProgramRunHandler final : public IProgramRunHandler, public TNonCopyable
     static constexpr std::string_view LogPrefix = "GenerateSql: ";
 
     const auto& settings = consumer-> GetPurecalcSettings();
-    const auto& filterExpr = consumer->GetFilterExpr();
+    auto filterExpr = TStringBuf(consumer->GetFilterExpr());
     const auto& watermarkExpr = consumer->GetWatermarkExpr();
 
     if (!filterExpr && !watermarkExpr) {
         LOG_ROW_DISPATCHER_TRACE("No sql was generated");
         return {};
     }
 
+    constexpr auto wherePrefix = "WHERE "sv;
+    if (filterExpr.starts_with(wherePrefix)) { // workaround for YQ-4827
+        filterExpr.remove_prefix(wherePrefix.size());
+    }
+
     using namespace fmt::literals;
     auto result = fmt::format(
         R"sql(

ydb/core/fq/libs/row_dispatcher/format_handler/ut/topic_filter_ut.cpp

@@ -329,7 +329,7 @@ Y_UNIT_TEST_SUITE(TestPurecalcFilter) {
         auto consumer = MakeConsumer(
             {{"a1", "[DataType; String]"}, {"a2", "[DataType; Uint64]"}, {"a@3", "[OptionalType; [DataType; String]]"}},
             "",
-            "a2 > 100",
+            "WHERE a2 > 100", // combined test for backward compatibility YQ-4827
             [&](ui64 offset, bool filter, TMaybe<ui64> watermark) {
                 offsets.push_back(offset);
                 filters.push_back(filter);