cherrylzhao opened a new issue #10150:
URL: https://github.com/apache/shardingsphere/issues/10150
### current logic
think we have a t_order table, 3 records distributed in 3 datasources as
follow:
```
+--------------------+---------+--------+
| order_id | user_id | status |
+--------------------+---------+--------+
| 591613421079212032 | jerry | init | -> ds0
| 591652652161937408 | jerry | init | -> ds1
| 591652696403456001 | jerry | init | -> ds2
+--------------------+---------+--------+
```
query sql is:
```
select user_id, count(user_id) from t_order group by user_id;
```
inside the shardingsphere, modified sql will be sent to backend datasources
to execute as follow:
```
+-----------------+-------------------------------------------------------------------------------------+
| datasource_name | sql
|
+-----------------+-------------------------------------------------------------------------------------+
| ds_0 | select user_id,count(user_id) from t_order group by
user_id ORDER BY user_id ASC |
| ds_1 | select user_id,count(user_id) from t_order group by
user_id ORDER BY user_id ASC |
| ds_2 | select user_id,count(user_id) from t_order group by
user_id ORDER BY user_id ASC |
+-----------------+-------------------------------------------------------------------------------------+
```
then 3 query results will be loaded into GroupByStreamMergeResult,
after MergeResult.next() was invoked core data flow is like this:
```
+-------------------------+
| currentRow |
+-------------------------+
[jerry, 1] -> count:1
[jerry, 1, jerry, 1] -> count:2
[jerry, 1, jerry, 1, jerry, 1] -> count:3
[jerry, 3, jerry, 1, jerry, 1] -> write count to currentRow
```
### optimize point
key point is that, count value was computed by `AggravationUnit` in
iteration process,
then update the currentRow value according to parsed aggregate projection
index after `group by` value changed.
so we should only cache first row of same group in currentRow to **reduce
memory used** like this:
```
+-------------------------+
| currentRow |
+-------------------------+
[jerry, 1] -> count:1
[jerry, 1] -> count:2
[jerry, 1] -> count:3
[jerry, 3] -> write count to currentRow
```
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