OpenTenBase / OpenTenBase/TXSQL
TXSQL count(expr)优化
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- Dominant language
- C++
- Stars
- 337
- Forks
- 234
- PR merge metrics
- No merged PRs in 30d
Description
在TXSQL中，count star会转换为count(0)，count star、count(0)、count(1)等价，表示计数时包含null值。而count(col)则不包含null值。
在存储引擎，count(col)需要转换col列到TABLE::record[0]中，因此存在格式转换开销。而count(0)不存在额外需要拷贝的列。
在SQL engine，通过Item判断nullable属性以及is_null属性。
如果count(expr)中的expr能够在优化阶段提前确定绝对不会为null，比如count(t.col),t不是left join内表，同时col定义为not null。此时
count(expr)实际可以转换为count star，从而也当作count(0)处理。从而可以减少格式转换开销。
下面是一个简单的测试：其中l_shipdate列定义为not null，而格式转换开销的消除使得查询变快了15%+。
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Research direction
Start by tracing count(expr) handling through the SQL engine's Item nullable and is_null checks, then follow how the storage engine converts columns into TABLE::record[0]. Verify the behavior for a NOT NULL column outside the inner side of a LEFT JOIN, and compare it with count(*) or count(0). Done means the eligible query path avoids the column conversion overhead and reproduces the reported performance improvement.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp, sql
- Domain
- databases, performance
- Issue type
- Feature
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 52/100