PostgreSQL source cancel query comments

src/Processors/Sources/PostgreSQLSource.cpp

@@ -193,7 +193,15 @@ PostgreSQLSource<T>::~PostgreSQLSource()
         {
             if (stream)
             {
+                /** Internally libpqxx::stream_from runs PostgreSQL copy query `COPY query TO STDOUT`.
+                  * During transaction abort we try to execute PostgreSQL `ROLLBACK` command and if
+                  * copy query is not cancelled, we wait until it finishes.
+                  */
                 tx->conn().cancel_query();
+
+                /** If stream is not closed, libpqxx::stream_from closes stream in destructor, but that way
+                  * exception is added into transaction pending error and we can potentially ignore exception message.
+                  */
                 stream->close();
             }