pingcap/tidb

Support spilling for distinct aggregate function

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#27,092 opened on Aug 11, 2021

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help wantedsig/executiontype/enhancement

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Description

Enhancement

Although we support spilling for unparallel HashAgg now, the memory usage of distinct is also out of control. The reason is that we need a IntSet/FloatSet/StringSet to check whether a value is distinct, and the Set maybe take up huge memory.

We need some spilling strategy for distinct function, to control the memory usage of SQL included distinct function.

Fortunately, only few aggregate function need to support distinct, so we only need consider the following distinct function: Count, Sum, Avg, GroupConcat

TiDB seems also support distinct for STDDEV_POP,STDDEV_SAMP, etc. But Mysql doesn't support the syntax

Contributor guide

Research direction
Investigate the existing spilling mechanism for HashAgg and extend it to handle distinct aggregate functions (Count, Sum, Avg, GroupConcat). Identify where the sets (IntSet, FloatSet, StringSet) are stored and how to spill them efficiently.
Tech stack
go
Domain
database
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Needs investigation
Prerequisites
GoSQLdatabase internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30