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Posted to commits@doris.apache.org by GitBox <gi...@apache.org> on 2020/10/26 05:23:27 UTC

[GitHub] [incubator-doris] wangbo opened a new issue #4788: [Proposal] Use Rollup's Aggregation Key as Rollup's Bucket Key

wangbo opened a new issue #4788:
URL: https://github.com/apache/incubator-doris/issues/4788

**Why Do So**
We observe that Doris's query performance is significantly slower that Kylin when the query contains bitmap computation.
The reason is that even a query hits rollup,doris's still need to do additional data scan and computation.
Doris's rollup use base table's distribution key which causes that rollup's bucket data may still has intersection.
Bitmap/HLL are greatly affected by this situation.

**Solution**
Use rollup's aggregation key as rollup's bucket key to make data truly pre-aggregated.

**POC**
env:
* 1 FE,3BE
* data:
* v2 storage format,one replica
* six bitmap column, each column's cardinality is about 5000,0000

Test result
* test sql: the sql completely hits rollup which contains six bitmap column
* case 1 : data has just load to Doris BE and not compaction completely
* rollup use base key as bucket key:
* query time:14s
* rollup use agg-key as buceck key:6s
* query time:6s
* case 2: data compaction completely
* rollup use base key as bucket key:
* first query time(without OS cache and BE's page cache):1.2s
* second querty time(hits be'a page cache): 1.0s
* scan bytes:241M
* scan rows: 1104
* return rows: 1104
* rollup use agg-key as buceck key:6s
* first query time(without OS cache and BE's page cache):1.2s
* second querty time(hits be'a page cache): 1.0s
* scan bytes:662M
* scan rows: 10079
* return rows: 1104
* case 3: data consistent
* query result is same whether rollup use rollup's agg-key as bucket key or rollup use base table's distribution key as bucket key

So we can see that when using rollup's agg-key as bucket key by is about three times performance improve than using base table's distribution key.
And because of rollup is truely pre-aggreagtion, scan data and computation is reduced.

**Future Work**
* Stage 1 : Make this feature available in production env quickly
Make it a configurable property for OLAP table when user wants to use rollup key's agg-key as bucket key in stream load/spark load.
Even they can set rollup's bucket num.

* Stage 2: Support Schema Change
I prefer to support schema change for this feature in Spark Job.
The reason as below:
```Read Write Separation``` is a necessary feature for Doris.
This feature needs to shuffle data when doing schema change would have a greater impact on the stability for Doris
Especially for a big table.
I don't think Doris is good at and need to be good at long time shuffle.
So Spark is the best choice.

* Stage 3: Support Colocate Join
Rollup has different agg-key need to shuffle join when query.

* Stage 4: Support Materialized view
Need further research.

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