pingcap/tidb
GitHub で見るParallel evaluation for aggregate functions with distinct in hashAgg
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#17,075 opened on 2020年5月10日
help wantedsig/executiontype/enhancementtype/performance
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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.