MySQL search very slow, bad query or indexes?

Question

We have a table in an application, which is VASTLY HUGE. Easily millions of rows.

The structure is basically as follows:

SERIES_ID | YEAR | DAY_SINCE_EPOCH | HOUR | MINUTE | VALUE

We have indexes on YEAR and DAY_SINCE_EPOCH.

The problem is that certain kind of queries are very slow, such as:

SELECT
 ...
WHERE
   SERIES_ID = 3 AND
   DAY_SINCE_EPOCH < 16000 AND
   YEAR = 2012
ORDER BY 
   DAY_SINCE_EPOCH DESC,
   HOUR DESC,
   MINUTE DESC
LIMIT 1

This takes about 10 seconds in a table with 2M rows, and well over 20 seconds in a table with 18M rows.

The intent is to find the last record of series 3, before day 16000. The YEAR=2012 is there to speed up the lookup.

So I was wondering, do we have the indexes set up right? Perhaps it'd be faster without the year index? Or with added SERIES_ID index or something like that?

Or just restructuring the query would help?

Any idea how to speed up the search will be welcome!

Answer 1

该索引可能会有所帮助：

> create index on your_table(series_id, day_since_epoch, year);

Answer 2

Check these conditions

SERIES_ID = 3 AND
DAY_SINCE_EPOCH < 16000 AND
YEAR = 2012

when you are updating the table(eg. saving new records) then save the true/false info on a "bit" column then check for it just like

where checked=1 .....

 SERIES_ID | YEAR | DAY_SINCE_EPOCH | HOUR | MINUTE | VALUE | checked