How to auto-extract or index day from timestamp in Postgres?

Question

We have a Postgres table with a timestamp field created_at . On a regular basis, we need to find all the records with the day field of created_at being a certain number.

We can run a query like

select * from table where extract(day from created_at) = 3;

I suspect this isn't efficient, ie it's doing a full-table scan. If so, can I create an index somehow to make the above efficient?

If it's not possible, we can create a separate column called created_at_day and create an index on it.

So we can simply run the query like

select * from table where created_at_day = 3;

Let's say created_at can be updated. Whenever this happens, created_at_day should be updated, too.

Does Postgres provide any support to automatically keep created_at_day in sync with created_at ? If so, how?

Of course this can be done in the application logic. So whenever created_at is created or updated, we update the created_at_day column. But just wondering if there's an easier, automated way to do this.

Thanks

Answer 1

You can create an index on extract(day from created_at)

To see the difference:

Create a table

knayak=# create table t as select i ,now()::timestamp + interval '1 days' * i as created_at from generate_series(1,10000) as i;
SELECT 10000

Create normal index on created_at

knayak=# create index ind_created_at on t(created_at);
CREATE INDEX

knayak=# explain analyze select * from t where extract(day from created_at) = 3;
                                           QUERY PLAN
-------------------------------------------------------------------------------------------------
 Seq Scan on t  (cost=0.00..205.00 rows=50 width=12) (actual time=1.049..6.020 rows=328 loops=1)
   Filter: (date_part('day'::text, created_at) = '3'::double precision)
   Rows Removed by Filter: 9672
 Planning time: 0.392 ms
 Execution time: 6.070 ms
(5 rows)

Create index with extract

knayak=# drop index ind_created_at;
DROP INDEX
knayak=# create index ind_created_at on t( extract(day from created_at) );
CREATE INDEX
knayak=# explain analyze select * from t where extract(day from created_at) = 3;
                                                        QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on t  (cost=4.67..61.66 rows=50 width=12) (actual time=0.110..0.260 rows=328 loops=1)
   Recheck Cond: (date_part('day'::text, created_at) = '3'::double precision)
   Heap Blocks: exact=54
   ->  Bitmap Index Scan on ind_created_at  (cost=0.00..4.66 rows=50 width=0) (actual time=0.093..0.093 rows=328 loops=1)
         Index Cond: (date_part('day'::text, created_at) = '3'::double precision)
 Planning time: 0.316 ms
 Execution time: 0.314 ms
(7 rows)