Im having a performance issue on using union. I have two sql queries with same column results but from different tables:
SELECT COL1, COL2, COL3
FROM TABLE1
(需要大约4秒完成)和:
SELECT COL1, COL2, COL3
FROM TABLE2 INNER JOIN TABLE3 ON ID
这需要大约4秒完成。当我使用 < code> UNionion ALL 来完成这两个查询时,结果需要22秒才能完成。
是否有改进业绩的建议?
UNION 条款通过在选择条款上使用 DISTINCT 并创建一些临时表格来消除重复内容,这些表格是符合要求的昂贵任务。 您是否需要这些重复内容?
校对""http://blog.sql authority.com/2009/03/11/sql-server-difference- between-union-vs-union-s-union-all-optimal-proper-compararison/" rel=“nofollow”> Pinnal Dave 对使用UNION和UNION All进行详细分析,并分析执行计划。
我还想检查这些选择条款是否正确使用指数,但很难说没有详细查看表格结构。
尝试使用 UNion all - 它避免找到重复。 没有 UNion 和 DistinT , 不使用 UNion 和 DistinT , 是一种良好做法, 但您需要的除外 。
与业绩问题一样:衡量。
下一步是从数据库中获取执行计划,然后看看。如果你不理解计划,打开一个新问题。
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