Merge pull request #5271 from 4ertus2/some

Cache ExpressionBlockInputStream header

dbms/src/DataStreams/ExpressionBlockInputStream.cpp

@@ -9,6 +9,8 @@ ExpressionBlockInputStream::ExpressionBlockInputStream(const BlockInputStreamPtr
     : expression(expression_)
 {
     children.push_back(input);
+    cached_header = children.back()->getHeader();
+    expression->execute(cached_header, true);
 }
 
 String ExpressionBlockInputStream::getName() const { return "Expression"; }
@@ -23,9 +25,7 @@ Block ExpressionBlockInputStream::getTotals()
 
 Block ExpressionBlockInputStream::getHeader() const
 {
-    Block res = children.back()->getHeader();
-    expression->execute(res, true);
-    return res;
+    return cached_header.cloneEmpty();
 }
 
 Block ExpressionBlockInputStream::readImpl()

dbms/src/DataStreams/ExpressionBlockInputStream.h

@@ -30,6 +30,7 @@ protected:
 
 private:
     ExpressionActionsPtr expression;
+    Block cached_header;
 };
 
 }