Question

Action  Keyname Type    Unique  Packed  Column  Cardinality Collation   Null    Comment
 Edit    Drop   PRIMARY BTREE   Yes No  TickerID    23200   A       
Timestamp   13897209    A   
 Edit    Drop   TickerID    BTREE   No  No  TickerID    11737   A       
 Edit    Drop   Timestamp   BTREE   No  No  Timestamp   18  A       

~99,424,209 InnoDB  utf8_general_ci 5.1 GiB 

OK before I try to “optimize” this DB by deleting unneeded indexes I thought I’d ask here. This table gets hit a lot so I want to speed up insert performance. I read 5 articles on the net that seem to indicate that the single index on TickerID is redundant because the multiple index (TickerID,Timestamp) will be used if I ever run a query on just tickerID.

Occasionally, I will like to do EOD reporting, so that I might just do something like SELECT * WHERE Timestamp > Today() - 1 day or something like that. Do I need the timestamp index as well?

Answer 1

This index on tickerId, timestamp can be used by all queries which can use an index on tickerId only.

However, for a query like that:

SELECT  *
FROM    mytable
WHERE   Timestamp > Today() - 1

an index on (tickerId, timestamp) will not be used, since there is no equality filter on tickerId.

For this query, you should create an index on timestamp only or rewrite thу query:

SELECT  m.*
FROM    (
        SELECT  DISTINCT tickerId
        FROM    mytable
        ) md
JOIN    mytable m
ON      m.tickerId = md.tickerId
        AND m.timestamp > TODAY() - 1

However, the latter query is less efficient, especially if you have lots of distinct values in tickerId.