Try make cache for getSampleBlock (#2313)

* Cache for getSampleBlock

* Update InterpreterSelectWithUnionQuery.cpp

* Update 00632_get_sample_block_cache.sql

* Add tests

dbms/src/Interpreters/Context.cpp

@@ -1732,6 +1732,11 @@ void Context::setFormatSchemaPath(const String & path)
     shared->format_schema_path = path;
 }
 
+Context::getSampleBlockCacheType & Context::getSampleBlockCache() const
+{
+    return getQueryContext().get_sample_block_cache;
+}
+
 std::shared_ptr<ActionLocksManager> Context::getActionLocksManager()
 {
     auto lock = getLock();
@@ -1742,7 +1747,6 @@ std::shared_ptr<ActionLocksManager> Context::getActionLocksManager()
     return shared->action_locks_manager;
 }
 
-
 SessionCleaner::~SessionCleaner()
 {
     try

dbms/src/Interpreters/Context.h

@@ -10,6 +10,7 @@
 #include <common/MultiVersion.h>
 #include <Core/Types.h>
 #include <Core/NamesAndTypes.h>
+#include <Core/Block.h>
 #include <Interpreters/Settings.h>
 #include <Interpreters/ClientInfo.h>
 #include <IO/CompressionSettings.h>
@@ -398,6 +399,10 @@ public:
     /// User name and session identifier. Named sessions are local to users.
     using SessionKey = std::pair<String, String>;
 
+    using getSampleBlockCacheType = std::unordered_map<std::string, Block>;
+    mutable Context::getSampleBlockCacheType get_sample_block_cache;
+    getSampleBlockCacheType & getSampleBlockCache() const;
+
 private:
     /** Check if the current client has access to the specified database.
       * If access is denied, throw an exception.

dbms/src/Interpreters/InterpreterSelectWithUnionQuery.cpp

@@ -9,7 +9,7 @@
 #include <DataTypes/getLeastSupertype.h>
 #include <Columns/ColumnConst.h>
 #include <Common/typeid_cast.h>
-
+#include <Parsers/queryToString.h>
 
 namespace DB
 {
@@ -157,7 +157,15 @@ Block InterpreterSelectWithUnionQuery::getSampleBlock(
     const ASTPtr & query_ptr,
     const Context & context)
 {
-    return InterpreterSelectWithUnionQuery(query_ptr, context).getSampleBlock();
+    auto & cache = context.getSampleBlockCache();
+    /// Using query string because query_ptr changes for every internal SELECT
+    auto key = queryToString(query_ptr);
+    if (cache.find(key) != cache.end())
+    {
+        return cache[key];
+    }
+
+    return cache[key] = InterpreterSelectWithUnionQuery(query_ptr, context).getSampleBlock();
 }
 
 

dbms/tests/queries/0_stateless/00632_get_sample_block_cache.reference

@@ -0,0 +1,2 @@
+Still alive
+65535