Question

I am trying to optimize this query as good as possible,but still i am getting query locks due to this query.Can any one provide some suggestions in improving it.The query fetches the last one day entries from the table.

The QUERY:

SELECT CR.id,
       CR.servicecode,
       CR.leadtime,
       CR.redirecturl,
       CRE.custemail,
       CRE.custlname,
       CRE.custfname,
       CRE.duration,
       CR.userid,
       AA.lpintrotimearr,
       AA.lpintrotimedep,
       AA.landdatetimearr,
       AA.landdatetimedep,
       CR.newcustid,
       cre.CRE.custmobilephone,
       CRE.brandname
FROM   response CR
       LEFT JOIN agreement AA
              ON CR.id = AA.id
       LEFT JOIN request CRE
              ON CRE.id = CR.id
WHERE  CR.id > '20120617145243'
       AND CR.approved = 1
       AND CR.chlapproved != 0
       AND CR.chlapproved IS NOT NULL
       AND AA.id IS NOT NULL
       AND ( AA.stdsign != 'on'
              OR AA.stdsign IS NULL )
       AND ( AA.ivaflag = 0
              OR AA.ivaflag IS NULL )
       AND ( AA.opt IS NULL
              OR AA.opt = 0 );

The EXPLAIN:

One way is to index all 3(AA.stdsign,AA.ivaflag and AA.opts) columns but all the three flags (AA.stdsign,AA.ivaflag and AA.opts) can have only 3 different values.Will indexing these reduce query run time?

All the ids are of varchar(60) data type.

Answer 1

There isn’t much to be improved on the query itself.
On the other hand, setting an index on AA.stdsign, AA.ivaflag and AA.opts should help a lot.

As your EXPLAIN indicates, no suitable key is found for your AA table and all 534956 rows must be scanned to satisfy the WHERE clause.

[edit]
One last tip: using large column types (such as VARCHAR(60)) for your primary keys is probably sub-optimal.

First reason: every time you need to reference a row (e.g. in a foreign key), you need another VARCHAR(60).

Second reason: comparisons on strings are slower than on integers (hence it may render a JOIN slower than necessary)

You may want to add an INT column to your tables, and use it as primary key.