Added more comments to AggregatingTransform.

src/Processors/Transforms/AggregatingTransform.cpp

@@ -413,6 +413,9 @@ AggregatingTransform::~AggregatingTransform() = default;
 
 IProcessor::Status AggregatingTransform::prepare()
 {
+    /// There are one or two input ports.
+    /// The first one is used at aggregation step, the second one - while reading merged data from ConvertingAggregated
+
     auto & output = outputs.front();
     /// Last output is current. All other outputs should already be closed.
     auto & input = inputs.back();

src/Processors/Transforms/AggregatingTransform.h

@@ -49,6 +49,21 @@ struct ManyAggregatedData
 using AggregatingTransformParamsPtr = std::shared_ptr<AggregatingTransformParams>;
 using ManyAggregatedDataPtr = std::shared_ptr<ManyAggregatedData>;
 
+/** Aggregates the stream of blocks using the specified key columns and aggregate functions.
+  * Columns with aggregate functions adds to the end of the block.
+  * If final = false, the aggregate functions are not finalized, that is, they are not replaced by their value, but contain an intermediate state of calculations.
+  * This is necessary so that aggregation can continue (for example, by combining streams of partially aggregated data).
+  *
+  * For every separate stream of data separate AggregatingTransform is created.
+  * Every AggregatingTransform reads data from the first port till is is not run out, or max_rows_to_group_by reached.
+  * When the last AggregatingTransform finish reading, the result of aggregation is needed to be merged together.
+  * This task is performed by ConvertingAggregatedToChunksTransform.
+  * Last AggregatingTransform expands pipeline and adds second input port, which reads from ConvertingAggregated.
+  *
+  * Aggregation data is passed by ManyAggregatedData structure, which is shared between all aggregating transforms.
+  * At aggregation step, every transform uses it's own AggregatedDataVariants structure.
+  * At merging step, all structures pass to ConvertingAggregatedToChunksTransform.
+  */
 class AggregatingTransform : public IProcessor
 {
 public: