diff --git a/dbms/src/Interpreters/Aggregator.cpp b/dbms/src/Interpreters/Aggregator.cpp
index 963184eac7b..d86ec714294 100644
--- a/dbms/src/Interpreters/Aggregator.cpp
+++ b/dbms/src/Interpreters/Aggregator.cpp
@@ -501,6 +501,7 @@ Block Aggregator::convertToBlock(AggregatedDataVariants & data_variants)
 		AggregatedDataWithUInt64Key & data = data_variants.key64;
 
 		IColumn & first_column = *key_columns[0];
+		first_column.reserve(data.size() + with_totals);
 
 		size_t j = with_totals ? 1 : 0;
 		for (AggregatedDataWithUInt64Key::const_iterator it = data.begin(); it != data.end(); ++it, ++j)
@@ -515,6 +516,7 @@ Block Aggregator::convertToBlock(AggregatedDataVariants & data_variants)
 	{
 		AggregatedDataWithStringKey & data = data_variants.key_string;
 		IColumn & first_column = *key_columns[0];
+		first_column.reserve(data.size() + with_totals);
 
 		size_t j = with_totals ? 1 : 0;
 		for (AggregatedDataWithStringKey::const_iterator it = data.begin(); it != data.end(); ++it, ++j)
@@ -529,6 +531,9 @@ Block Aggregator::convertToBlock(AggregatedDataVariants & data_variants)
 	{
 		AggregatedDataWithKeys128 & data = data_variants.keys128;
 
+		for (size_t i = 0; i < keys_size; ++i)
+			key_columns[i]->reserve(data.size() + with_totals);
+
 		size_t j = with_totals ? 1 : 0;
 		for (AggregatedDataWithKeys128::const_iterator it = data.begin(); it != data.end(); ++it, ++j)
 		{
@@ -548,6 +553,9 @@ Block Aggregator::convertToBlock(AggregatedDataVariants & data_variants)
 	{
 		AggregatedDataHashed & data = data_variants.hashed;
 
+		for (size_t i = 0; i < keys_size; ++i)
+			key_columns[i]->reserve(data.size() + with_totals);
+
 		size_t j = with_totals ? 1 : 0;
 		for (AggregatedDataHashed::const_iterator it = data.begin(); it != data.end(); ++it, ++j)
 		{