Auto distributed_group_by_no_merge on GROUP BY injective function of sharding key

src/Storages/StorageDistributed.cpp

@@ -17,6 +17,7 @@
 #include <Common/Macros.h>
 #include <Common/escapeForFileName.h>
 #include <Common/typeid_cast.h>
+#include <Common/quoteString.h>
 
 #include <Parsers/ASTDropQuery.h>
 #include <Parsers/ASTExpressionList.h>
@@ -73,6 +74,7 @@ namespace ErrorCodes
     extern const int NO_SUCH_COLUMN_IN_TABLE;
     extern const int TOO_MANY_ROWS;
     extern const int UNABLE_TO_SKIP_UNUSED_SHARDS;
+    extern const int LOGICAL_ERROR;
 }
 
 namespace ActionLocks
@@ -378,8 +380,54 @@ StoragePtr StorageDistributed::createWithOwnCluster(
 }
 
 
+bool StorageDistributed::canForceGroupByNoMerge(const Context &context, const ASTPtr & query_ptr) const
+{
+    const auto & settings = context.getSettingsRef();
+
+    if (settings.distributed_group_by_no_merge)
+        return true;
+    if (!settings.optimize_skip_unused_shards)
+        return false;
+    if (!has_sharding_key)
+        return false;
+
+    const auto & select = query_ptr->as<ASTSelectQuery &>();
+
+    if (select.orderBy())
+        return false;
+    if (select.distinct)
+        return false;
+
+    // This can use distributed_group_by_no_merge but in this case limit stage
+    // should be done later (which is not the case right now).
+    if (select.limitBy() || select.limitLength())
+        return false;
+
+    const ASTPtr group_by = select.groupBy();
+    if (!group_by)
+        return false;
+
+    // injective functions are optimized out in optimizeGroupBy()
+    // hence all we need to check is that column in GROUP BY matches sharding expression
+    auto & group_exprs = group_by->children;
+    if (!group_exprs.size())
+        throw Exception("No ASTExpressionList in GROUP BY", ErrorCodes::LOGICAL_ERROR);
+
+    auto id = group_exprs[0]->as<ASTIdentifier>();
+    if (!id)
+        return false;
+    if (!sharding_key_expr->getSampleBlock().has(id->name))
+        return false;
+
+    LOG_DEBUG(log, "Force distributed_group_by_no_merge for GROUP BY " << backQuote(serializeAST(*group_by, true)) << " (injective)");
+    return true;
+}
+
 QueryProcessingStage::Enum StorageDistributed::getQueryProcessingStage(const Context &context, QueryProcessingStage::Enum to_stage, const ASTPtr & query_ptr) const
 {
+    if (canForceGroupByNoMerge(context, query_ptr))
+        return QueryProcessingStage::Complete;
+
     auto cluster = getOptimizedCluster(context, query_ptr);
     return getQueryProcessingStageImpl(context, to_stage, cluster);
 }

src/Storages/StorageDistributed.h

@@ -67,6 +67,8 @@ public:
 
     bool isRemote() const override { return true; }
 
+    /// Return true if distributed_group_by_no_merge may be applied.
+    bool canForceGroupByNoMerge(const Context &, const ASTPtr &) const;
     QueryProcessingStage::Enum getQueryProcessingStage(const Context &, QueryProcessingStage::Enum /*to_stage*/, const ASTPtr &) const override;
 
     Pipes read(