pingcap/tidb

TiDB gets row data even using covered index

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#37,765 opened on Sep 13, 2022

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help wantedsig/transactiontype/enhancement

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Description

Enhancement

create table t1(id1 int, id2 int, id3 int, PRIMARY KEY(id1), UNIQUE KEY udx_id2 (id2));

SELECT id2 FROM t1 WHERE id2 = 1

The point-get statement above used covered index udx_id2 , but TiDB still needs to get the row data. In fact, we should remove this step.

Contributor guide

Research direction
Investigate the point get execution path in TiDB's storage layer to identify why row data is fetched despite using a covering index. Look for code that handles unique index lookups and see if there is an unconditional row retrieval after index lookup.
Tech stack
go
Domain
backenddatabase
Issue type
Bug
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.
Half day
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.
Clear
Prerequisites
GoSQLTiDB internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40