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Descriptions of Variables > Performance-Related Options
Performance-Related Options
Here I’ll offer a short overview of options that affect performance. Again, I am not describing all of them, but just those that are used most frequently. In contrast with options from the previous section, these options don’t lead to different results.[11]
First I will discuss options that affect server behavior as a whole, then some engine-specific options.
The first group of options controls the amount of memory used internally by the server and certain upper limits on memory usage.
join_buffer_size-
This is the minimum size of the buffer allocated for joins that use plain index scans, and range scans, and joins that do not use indexes. The buffer is allocated for each full join between two tables. Thus, a query joining two tables allocates one buffer, a query joining three tables allocates two buffers, and so on. This can be used as a session variable and can be set for a particular join.
