Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Ask a Question
Welcome To Ask or Share your Answers For Others

Categories

sql server - performance of union versus union all

0 votes
707 views
in Technique[技术] by (71.8m points)

sql server - performance of union versus union all

I have to run a select statement across several tables. I am sure the tables return different records. I am anyway using UNION ALL.

Is it better to use UNION or of UNION ALL in performance terms when I am sure the tables return different records?

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Answer

0 votes
by (71.8m points)

UNION ALL will perform better than UNION when you're not concerned about eliminating duplicate records because you're avoiding an expensive distinct sort operation. See: SQL SERVER – Difference Between Union vs. Union All – Optimal Performance Comparison


与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

Just Browsing Browsing

2.1m questions

2.1m answers

60 comments

57.0k users

Most popular tags

Powered by Question2Answer
Donate
Theme by Q2A Market&&OStack.cn
...