pingcap/tidb

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

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#12,723 创建于 2019年10月15日

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描述

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:

贡献者指南

技术栈
gosql
领域
database
议题类型
feature
难度面向新贡献者的预计实现难度，1 表示很小改动，5 表示专家级工作。
4
预计时间有经验贡献者完成调查、实现、测试并准备 pull request 的粗略时间范围。
3-5 days
活动状态议题当前的可参与程度：新鲜、活跃、陈旧、阻塞或等待维护者输入。
stale
清晰度议题是否清楚说明期望改动、验收标准和下一步。
clear
前置要求
Go programmingUnderstanding of query execution in TiDBKnowledge of correlated subqueries
新手友好度1-100 的估计分数，表示该议题对首次贡献者的友好程度。
20
研究方向
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.