A few customers with rather extreme needs have contacted us about a performance issue with the range optimizer. Our solution to the problem is to introduce a new variable in MySQL 5.6, eq_range_index_dive_limit, which can be used to control whether or not the range optimizer will a) do index dives, or b) use index statistics when estimating the number of rows in the ranges of the query. The former method gives a far more accurate estimate while the latter costs a lot less to compute.

This is what the help text has to tell about the variable:

The optimizer will use existing index statistics instead of doing index dives for equality ranges if the number of equality ranges for the index is larger than or equal to [the value of variable]. If set to 0, index dives are always used."Equality range" means predicates using operators IN() or =, and it's important to notice that the number of such ranges is counted on a per index …

Over the last few weeks I’ve been looking at several customers’ slow query logs, and I found in many of them an odd type of query. These are SELECT statements that contain an IN clause that includes dozens, sometimes hundreds of values. These statements often end in the slow query log. I’m not sure if these queries are this way by design or if they are generated by a specific database development tool.

I did some tests in one of my own databases, one with only around 10K rows in its largest table. The database corresponds to the Amarok media player. For example, I queried for songs by B. B. King (spelled “BB King”, “B.B. King”, etc. or with other artists: “B. B. King & Eric Clapton”).

The first query used a JOIN and an IN clause with all the …