MySQL查询太慢

Question

at a MySQL database I have the following table:

• id int Primary key
• timestamp timestamp
• tpid varchar
• tpidno int
• serialnumber int
• command varchar
• sequence int
• startTime varchar
• endTime varchar
• PosData varchar
• ...

I also have 3 secondary indices:

1. tpid,tpidno
2. serialnumber
3. command

The table contains ~2.5M rows and it is about 500MB

Although I have complex queries that work fast I have great delay on those two simple queries:

Select id, sequence, PosData 
  From myTable 
 Where  serialNumber = 130541  
   and command = "myCommand" 
   and startTime = "20140106194300" 
   and endtime = "20140106200000"

(~4.4sec)

Select id 
  From myTable 
 Where  serialNumber = 130541  
   and command = 'myCommand' 
   and sequence = 128

(~4.5sec)

Does more indices like

• serialnumber, command
• command, sequence

or

• serialnumber,command, sequence

will speed up the queries?

At the first query is it possible the data type of startTime and endTime to be the problem? if they were int instead of varchar it would be better?

any other suggestions?

Answer 1

A single index on (serialnumber, command) will definitively improve performance for those two queries. Further you could add the other columns to make it even faster. However, the best choice of the other columns depends on the data distribution and on the question which of the two statements is more often executed. It might even not worth adding those columns if the other two columns are very selective.

The datatype for startdate and enddate is unfortunate at least. Proper types will improve performance in a range from "a little" to "a lot" depending on you SQL. The SQL above will be in the "a little" range.

Some refs:

• How multi-column indexes work
• Possible problems when using improper types (ex: varchar instead of numeric types)