This is my analytics database (not dev) so no extrapolation is necessary except in that I know the tables will grow in size. The database is hosted on AWS and maintained by Heroku.
On Fri, Feb 17, 2012 at 11:21 AM, Steve Crawford <scrawford@xxxxxxxxxxxxxxxxxxxx> wrote:
On 02/17/2012 10:34 AM, Alessandro Gagliardi wrote:
Though I could figure it out, it would be helpful to actually specify which query is faster and to post the explain of *both* queries.Comparing...
SELECT DISTINCT(user_id) FROM blocks JOIN seen_its USING (user_id) WHERE seen_its.created BETWEEN (now()::date - interval '8 days')::timestamp AND now()::date::timestamp
to
SELECT DISTINCT(user_id) FROM seen_its WHERE created BETWEEN (now()::date - interval '8 days')::timestamp AND now()::date::timestamp
the difference is 100x.
But in general, it is not terribly unusual to find that rewriting a query can lead the planner to generate a superior plan. Trying and testing different ways of writing a query is a standard tuning technique.
There are also version-specific issues with some versions of PostgreSQL preferring ...where foo in (select... and others preferring ...where exists (select...
If you are planning to ramp up to high volumes it is also *very* important to test and tune using the size of database you plan to have on the hardware you will use in production. You cannot extrapolate from a dev database on an i486 (?!?) machine to a production server with more spindles, different RAID setup, different CPU, more cores, vastly more memory, etc.
In the case of your queries, the second one eliminates a join and gives the planner an easy way to optimize using the available indexes so I'm not surprised it's faster.
Note: I am guessing that your seen_its table just grows and grows but is rarely, if ever, modified. If it is basically a log-type table it will be a prime candidate for partitioning on date and queries like this will only need to access a couple relatively small child tables instead of one massive one.
Cheers,
Steve
