Mysql improve SELECT speed

Question

I'm currently trying to improve the speed of SELECTS for a MySQL table and would appreciate any suggestions on ways to improve it.

We have over 300 million records in the table and the table has the structure tag, date, value. The primary key is a combined key of tag and date. The table contains information for about 600 unique tags most containing an average of about 400,000 rows but can range from 2000 to over 11 million rows.

The queries run against the table are:

  SELECT date,
         value 
    FROM table 
   WHERE tag = "a" 
     AND date BETWEEN 'x' and 'y' 
ORDER BY date

....and there are very few if any INSERTS.

I have tried partitioning the data by tag into various number of partitions but this seems to have little increase in speed.

Answer 1

What is the cardinality of the date field (that is, how many different values appear in that field)? If the date BETWEEN 'x' AND 'y' is more limiting than the tag = 'a' part of the WHERE clause, try making your primary key (date, tag) instead of (tag, date), allowing date to be used as an indexed value.

Also, be careful how you specify 'x' and 'y' in your WHERE clause. There are some circumstances in which MySQL will cast each date field to match the non-date implied type of the values you compare to.

Answer 2

Your query is asking for a few things - and with that high # of rows, the look of the data can change what the best approach is.

   SELECT date, value 
   FROM table 
   WHERE tag = "a" 
     AND date BETWEEN 'x' and 'y' 
   ORDER BY date

There are a few things that can slow down this select query.

1. A very large result set that has to be sorted (order by).
2. A very large result set. If tag and date are in the index (and let's assume that's as good as it gets) every result row will have to leave the index to lookup the value field. Think of this like needing the first sentence of each chapter of a book. If you only needed to know the chapter names, easy - you can get it from the table of contents, but since you need the first sentence you have to go to the actual chapter. In certain cases, the optimizer may choose just to flip through the entire book (table scan in query plan lingo) to get those first sentences.
3. Filtering by the wrong where clause first. If the index is in the order tag, date... then tag should (for a majority of your queries) be the more stringent of the two columns. So basically, unless you have more tags than dates (or maybe than dates in a typical date range), then dates should be the first of the two columns in your index.

A couple of recommendations:

1. Consider if it's possible to truncate some of that data if it's too old to care about most of the time.
2. Try playing with your current index - i.e. change the order of the items in it.
3. Do away with your current index and replace it with a covering index (has all 3 fields in it)
4. Run some EXPLAIN's and make sure it's using your index at all.
5. Switch to some other data store (mongo db?) or otherwise ensure this monster table is kept as much in memory as possible.