如何优化慢速Mysql SELECT查询？

Question

Here is my query:

SELECT col1, col2, col3, col4, col5, col6,col7, col8,..,col18
FROM table1
WHERE col1= 'val' and col7='Y' and col16='203' OR col16='201' order by col4 desc

I don't know what is making this query slow, whether it's the order by ,or the where clauses...

Properely added the index also, but it's still slow.

I'am using JSP + STRUTS + EJB2.0 + MYSQL. The table1 has more than half million records. How can I optimize the query or what are the other possibilities to improve the speed of execution?

table structure

col1                varchar(20) NO  PRI 
col2                varchar(50) NO  PRI 
col3                varchar(50) YES     [NULL]
col4                varchar(20) YES     [NULL]
col5                varchar(6)  YES     [NULL]
col6                varchar(20) YES     [NULL]
col7                varchar(1)  YES     [NULL]
col8               mediumtext   YES     [NULL]
col9              mediumtext    YES     [NULL]
col10             mediumtext    YES     [NULL]
col11              mediumtext   YES     [NULL]
col12              mediumtext   YES     [NULL]
col13               mediumtext  YES     [NULL]
col14             mediumtext    YES     [NULL]
col15               mediumtext  YES     [NULL]
col16               varchar(20) YES     [NULL]
col17            varchar(50)    YES     [NULL]
col18             varchar(5)    YES     [NULL]
col19              varchar(5)   YES     [NULL]
col20               varchar(5)  YES     [NULL]
col21                  text YES     [NULL]
col 22                  text    YES     [NULL]
col23              text YES     [NULL]
col24              varchar(5)   YES     [NULL]
col25              int(11)  YES     [NULL]

Answer 1

I don't know what it is making this query slow, whether order by, or conditions..

Half a million records should fit in memory if they're the usual sort of name, phone nr, email sort of thing (and not documents). So if it's -very- slow, something is very wrong.

properly added index also,still slow.

Which columns are indexed? You need to index by the column that will filter most effectively. For example, if col2 is the answer to a yes or no question, it won't help to index by that.

Answer 2

Your query -

SELECT col1, col2, col3, col4, col5, col6,col7, col8,..,col18  
FROM table1 
where 
col1= 'val' 
and col2='Y' 
and (col3='203' OR col3='201')
order by col4 desc

This will first need a covering index

alter table table1 add index search_idx(col1,col2,col3) ;

Now to resolve the order by clause you will need to index that as well

alter table table1 add index col4_idx(col4) ;

Now note that the or condition is killer and performace wise its better to convert this to union all

SELECT col1, col2, col3, col4, col5, col6,col7, col8,..,col18  
FROM table1 
where 
col1= 'val' 
and col2='Y' 
and col3='203'
union all
SELECT col1, col2, col3, col4, col5, col6,col7, col8,..,col18  
FROM table1 
where 
col1= 'val' 
and col2='Y' 
and col3='201'
order by col4 desc

You may use explain select for the above queries to analyze the query health.

Make sure to take a backup of the table before applying the index.

https://dev.mysql.com/doc/refman/5.0/en/select-optimization.html

https://dev.mysql.com/doc/refman/5.0/en/order-by-optimization.html

Answer 3

Are you sure your WHERE-condition is correct?

AND takes precedence over OR, so

where col1= 'val' and col2='Y' and col3='203' OR col3='201'

is the same as

where (col1= 'val' and col2='Y' and col3='203') OR (col3='201')

But you probably want

where col1= 'val' and col2='Y' and (col3='203' OR col3='201')

Edit: Based on your comment my assumption was wrong and you actually want the original result (Then I would suggest adding parentheses to make it clear). The only possible index in that case would be on col3 (if it's selective enough).

I don't know if mysql's optimizer is smart enough to access a table using the same index twice, if not you need to UNION ALL two queries:

SELECT col1, col2, col3, col4, col5, col6,col7, col8,..,col18
FROM table1
WHERE col1= 'val' AND col2='Y' AND col3='203'

UNION ALL

SELECT col1, col2, col3, col4, col5, col6,col7, col8,..,col18
FROM table1
WHERE col3='201'
ORDER BY col4 DESC

Edit2: After the OP edited the question the column names are wrong ( col2 will be col7 and col3 will be col16 )