Question

I’m trying to optimise my query, it has an inner join and coalesce.

The join table, is simple a table with one field of integer, I’ve added a unique key.

For my where clause I’ve created a key for the three fields.

But when I look at the plan it still says it’s using a table scan.

Where am I going wrong ?

Here’s my query

select date(a.startdate, '+'||(b.n*a.interval)||' '||a.intervaltype) as due

from billsndeposits a 

inner join util_nums b on date(a.startdate, '+'||(b.n*a.interval)||' 
     '||a.intervaltype) <= coalesce(a.enddate, date('2013-02-26')) 

where not (intervaltype = 'once' or interval = 0) and factid = 1 

order by due, pid;

Answer 1

I think I understand what you are trying to achieve. But this kind of join is a recipe for slow performance. Even if you remove date computations and the coalesce (i.e. compare one date against another), it will still be slow (compared to integer joins) even with an index. And because you are creating new dates on the fly you cannot index them.

I suggest creating a temp table with 2 columns (1) pid (or whatever id you use in billsndeposits) and (2) recurrence_dt

populate the new table using this query:

INSERT INTO TEMP
SELECT PID, date(a.startdate, '+'||(b.n*a.interval)||' '||a.intervaltype)
FROM billsndeposits a, util_numbs b;

Then create an index on recurrence_dt columns and runstats. Now your select statement can look like this:

SELECT recurrence_dt
FROM temp t, billsndeposits a
WHERE t.pid = a.pid
  AND recurrence_dt <= coalesce(a.enddate, date('2013-02-26'))

you can add a exp_ts on this new table, and expire temporary data afterwards.

I know this adds more work to your original query, but this is a guaranteed performance improvement, and should fit naturally in a script that runs frequently.

Regards,

Edit

Another thing I would do, is make enddate default value = date(‘2013-02-26’), unless it will affect other code and/or does not make business sense. This way you don’t have to work with coalesce.