性能：WHERE IN子句与（INSERT + INNER JOIN）

Question

I have a use-case where I need to perform a very high number of SELECT SQL

I have two approaches at this moment:

1. Query by a list of identifiers. So, I first used WHERE IN clause:

   • SELECT COL1, COL2, COL3, COL4 FROM MAIN_TABLE WHERE COL1 IN ( 1,2,3,8,11,78,59,65,74,25,36,54558,78854,558 )

2. I can create a table, let's say, CACHE_TABLE, and first INSERT the identifiers ( 1,2,3,8,11,78,59,65,74,25,36,54558,78854,558 ) into it by a unique key CACHEID and the JOIN this CACHE_TABLE with MAIN_TABLE to get the desired result:

   • SELECT MT.COL1, MT.COL2, MT.COL3, MT.COL4 FROM MAIN_TABLE MT JOIN CACHE_TABLE CT ON CT.IDENTIFIER = MT.COL1 WHERE CT.CACHEID =

Performance is really critical in my use-case. So I wanted to know if the approach #2 would yield better performance than approach #1. Also, if there is a better alternative approach(s) for this

Thanks a ton in Advance!!

Answer 1

your answer is best performance approach #2. In my experience IN is a very slow operator, since SQL normally evaluates it as a series of WHERE clauses separated by "OR" (WHERE x=Y OR x=Z OR...). As with ALL THINGS SQL though, your mileage may vary. The speed will depend a lot on indexes

Answer 2

You need to test the two approaches.

For a single query, I would expect in to win in most cases -- simply because creating the table and then uses it requires multiple round-trips to the database.

In addition, some databases optimize constant lists (for instance, MySQL does a binary search on values rather than a sequential search).

The one thing that will help either version is an index on (col1) or (col1, col2, col3, col4) .