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update tiflash expr pushdown list (#3780) #3782

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Merged
merged 3 commits into from
Jun 23, 2020
Merged

update tiflash expr pushdown list (#3780) #3782

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4a227ed
cherry pick #3780 to release-4.0
windtalker Jun 23, 2020
d17c5e6
fix conflict
windtalker Jun 23, 2020
1e2f7e6
Merge branch 'release-4.0' into release-4.0-253d66f08f92
windtalker Jun 23, 2020
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13 changes: 10 additions & 3 deletions tiflash/use-tiflash.md
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Expand Up @@ -222,11 +222,18 @@ TiSpark 目前提供类似 TiDB 中 engine 隔离的方式读取 TiFlash，方

TiFlash 主要支持谓词、聚合下推计算，下推的计算可以帮助 TiDB 进行分布式加速。暂不支持的计算类型主要是表连接和 DISTINCT COUNT，会在后续版本逐步优化。

目前 TiFlash 支持了有限的常用表达式下推，支持下推的表达式可参考[该文件](https://github.com/pingcap/tidb/blob/v4.0.0-rc/expression/expression.go#L979)。
目前 TiFlash 支持了有限的常用表达式下推，支持下推的表达式包括：

```
+, -, /, *, >=, <=, =, !=, <, >, ifnull, isnull, bitor, in, mod, bitand, or, and, like, not,
case when, month, substr, timestampdiff, date_format, from_unixtime, json_length, if, bitneg, bitxor, cast(int as decimal), date_add(datetime, int), date_add(datetime, string)
```

其中，`cast` 和 `date_add` 的下推默认不开启，若需要手动开启，请参考[优化规则及表达式下推的黑名单](/blacklist-control-plan.md)

目前 TiFlash 不支持下推的情况包括：

- 所有包含 Duration 和 JSON 的表达式均不能下推
- 在聚合函数或者 WHERE 条件中包含了不在[该文件](https://github.com/pingcap/tidb/blob/v4.0.0-rc/expression/expression.go#L979)列表中的表达式，聚合或者相关的谓词过滤均不能下推
- 所有包含 Duration 的表达式均不能下推
- 在聚合函数或者 WHERE 条件中包含了不在上述列表中的表达式，聚合或者相关的谓词过滤均不能下推

如查询遇到不支持的下推计算，则需要依赖 TiDB 完成剩余计算，可能会很大程度影响 TiFlash 加速效果。对于暂不支持的表达式，将会在后续陆续加入支持，也可以联系官方沟通。