pingcap/tidb

Parallel evaluation for aggregate functions with distinct in hashAgg

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#17,075 建立於 2020年5月10日

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help wantedsig/executiontype/enhancementtype/performance

描述

Development Task

select count(distinct a) from t;

Currently if one of the aggregate functions of a SQL has a distinct evaluation, the hashAgg executor must run in a single thread. See https://github.com/pingcap/tidb/blob/3224393/executor/builder.go#L1207

We may find a method to evaluate it in parallel, maybe a sync.Map for deduplicating entries can be shared in multiple goroutines.

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go
領域
backenddatabaseperformance
議題類型
feature
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前置要求
Goparallelismsync.Map
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30
研究方向
Examine the current implementation in executor/builder.go around line 1207. The approach involves using sync.Map for deduplication across goroutines. The main challenge is ensuring thread safe accumulation while maintaining correctness. Refer to the linked file and consider implementing a shared sync.Map for distinct values, then aggregating results after goroutines finish. Test with the provided SQL query.