如何优化SQL查询？

Question

I have a query:

SELECT * FROM `trades`
WHERE `symbol` = 'ICX/BTC' AND `timestamp` >= :since AND `timestamp` <= :until
ORDER BY `timestamp`
LIMIT 50000

It's take long time to execute (more 5 min). I have index by symbol and timestamp columns

How I can optimize this query?

Answer 1

For this query:

SELECT t.*
FROM trades AS t
WHERE t.symbol = 'ICX/BTC' AND t.timestamp >= :since AND t.timestamp <= :until
ORDER BY t.timestamp
LIMIT 50000;

(which I just rewrite a bit so I can follow it more easily)

You want an index on trades(symbol, timestamp) .

However, you appear to be selecting a very large number of rows, so this might still take a long time. The index should be used both for the WHERE clause and the ORDER BY .

Answer 2

In your query, you are retrieving data from just one table and your filter criteria are ...

1. equality on symbol

2. range scan low-to-high on timestamp .

Therefore, (as Gordon mentioned) an index on two columns (symbol, timestamp) can satisfy your query, both the filtering and the ordering, quite efficiently. The query planner will do a random access operation on the index to the correct symbol and the starting timestamp, then read the index sequentially until the ending timestamp. That's efficient.

But, your SELECT * may be holding you back on performance. Why? If you used, for example, SELECT symbol, timestamp, cusip, name then you could create a so-called covering index on (symbol, timestamp, cusip, name) . In that case, the entire query would be satisfied by scanning the index. That can be very efficient indeed.

Pro tip Avoid SELECT * , both for software stability and performance reasons.

Pro tip Don't add extra indexes to a table unless you know they will help particular queries. MySQL only uses a single index for each table in a query or subquery. Neither an index on just timestamp or just symbol will help much: MySQL still has to examine a lot of rows to satisfy your filtering criteria.