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Editorial Team
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Asked: 2026-05-18T06:50:31+00:00

In a tb with 1 mil. rows if I do (after I restart the

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In a tb with 1 mil. rows if I do (after I restart the computer – so nothing it’s cached):
1. SELECT price,city,state FROM tb1 WHERE zipId=13458;
the result is 23rows in 0.270s

after I run ‘LOAD INDEX INTO CACHE tb1’ (key_buffer_size=128M and total index size for tb is 82M):
2. SELECT price,city,state FROM tb1 WHERE zipId=24781;
the result is 23rows in 0.252s, Key_reads remains constant, Key_read_requests is incremented with 23

BUT after I load ‘zipId’ into OS cache, if I run again the query:
2. SELECT price,city,state FROM tb1 WHERE zipId=20548;
the result is 22rows in 0.006s

This it’s just a simple example, but I run tens of tests and combinations. But the results are always the same.
I use: MySql with MyISAM, WINDOWS 7 64, and the query_cache is 0;
zipId it’s a regular index (not primary key)

SHOULDN’T key_cache be faster than OS cache ??
SHOULDN’T be a huge difference in speed, after I load the index into cache ??
(in my test it’s almost no difference).

I’ve read a lot of websites,tutorials and blogs on this matter but none of them really discuss the difference in speed. So, any ideas or links will be greatly appreciated.
Thank you.

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1 Answer

  1. Editorial Team

    Under normal query processing, MySQL will scan the index for the where clause values (i.e. zipId = 13458). Then uses the index to look up the corresponding values from the MyISAM main table (a second disk access). When you load the table into memory, the disk accesses are all done in memory, not from reading a real disk.

    The slow part of the query is the lookup from the index into the main table. So loading the index into memory may not improve the query speed.

    One thing to try is Explain Select on your queries to see how the index is being used.

    Edit: Since I don’t think the answers to your comments will fit in a comment space. I’ll answer them here.

    MyISAM in and of itself does not have a cache. It relies upon the OS to do the disk caching. How much of your table is cached by depends upon what else you are running in the system, and how much data you are reading through. Windows in particular does not allow the user much control over what data is cached and for how long.

    The OS caches disk blocks (either 4K or 8K chunks) of the index file or the full table file.

    SELECT indexed_col FROM tb1 WHERE zipId+0>1

    Queries like this where you use functions on the predicate (Where clause) can cause MySQL to do full table scans rather than using any index. As I suggested above, use EXPLAIN SELECT to see what MySQL is doing.

    If you want more control over the cache, try using an INNODB table. The InnoDB engine creates its own cache which you can size, and does a better job of keeping the most recent used stuff in it.

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