pingcap/tidb

shortcut for selection's execution when its filter contain correlated filter.

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#12,723 opened on 2019年10月15日

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help wantedsig/executiontype/enhancementtype/performance

説明

Feature Request

Is your feature request related to a problem? Please describe:

For normal selection not involved in Apply. The constant filter will be evaled during planning. So if one filter would filter out all data, it will be converted to TableDual during planning. But if there's correlated column in filters, we would eval it in execution phase. Though it atually behaves like a constant in execution phase.

Describe the feature you'd like:

We could split the filters to two part and one part only contains filters containing constant and correlated column. Then when calling the Next of SelectionExec for the first time. We eval these filters once. If it's false. Then we can break the execution early.

Describe alternatives you've considered:

Currently, no

Teachability, Documentation, Adoption, Migration Strategy:

コントリビューターガイド

技術スタック
gosql
領域
database
Issue 種別
feature
難度新しい貢献者にとっての実装難度です。1 は非常に小さな変更、5 は専門的な作業です。
4
推定時間経験ある貢献者が調査、実装、テスト、pull request 準備にかけるおおよその時間範囲です。
3-5 days
活動状況Issue が今どれだけ取り組みやすいかを示します。新しい、活発、古い、ブロック中、またはメンテナー入力待ちなどです。
stale
明確さ期待される変更、受け入れ条件、次の手順がどれだけ明確かを示します。
clear
前提条件
Go programmingUnderstanding of query execution in TiDBKnowledge of correlated subqueries
初心者向け度初回貢献者にどれだけ取り組みやすいかを 1-100 で推定したスコアです。
20
調査方針
The issue proposes an optimization for SelectionExec when filters contain correlated columns. Currently, constant filters are evaluated during planning, but correlated filters are evaluated at execution. The idea is to split filters into two parts: one with constant and correlated columns, and evaluate them once on the first call to Next(). If false, break early. To implement this, study the SelectionExec code in the TiDB repository (likely in executor/selection.go). Understand how filters are currently handled and where to add the early evaluation logic. No linked PRs or comments provide further guidance, so the implementation should follow the described approach.