PostgreSQL Indexing Run Time

Question

Supposing I have this table with index.

 Create Table sample
 (
    table_date timestamp without timezone,
    description
 );
  CREATE INDEX dailyinv_index
  ON sample
  USING btree
  (date(table_date));

And It has a 33 million rows.

Why is it that running this query

   select count(*) from sample where date(table_date) = '8/30/2017' and   desc = 'desc1'

yields a result @ 12ms

Using PostgreSQL to explain the query plan. this is what it does.

    Aggregate  (cost=288678.55..288678.56 rows=1 width=0)
      ->Bitmap Heap Scan on sample (cost=3119.63..288647.57 rows=12393 width=0)
           Recheck Cond: (date(table_date) = '2017-08-30'::date)
           Filter: ((description)::text = 'desc1'::text)
               ->  Bitmap Index Scan on dailyinv_index  (cost=0.00..3116.54 rows=168529 width=0)
                     Index Cond: (date(table_date) = '2017-08-30'::date)

but this one

   select date(table_date) from sample where date(table_date)<='8/30/2017' order by table_date desc limit 1

yields result after 11,460 ms?

Query Plan

   Limit  (cost=798243.52..798243.52 rows=1 width=8)
     ->  Sort  (cost=798243.52..826331.69 rows=11235271 width=8)
             Sort Key: table_date
                  ->  Bitmap Heap Scan on sample  (cost=210305.92..742067.16 rows=11235271 width=8)
                        Recheck Cond: (date(table_date) <= '2017-08-30'::date)
                             ->  Bitmap Index Scan on dailyinv_index  (cost=0.00..207497.10 rows=11235271 width=0)
                                    Index Cond: (date(table_date) <= '2017-08-30'::date)

PostgreSQL Version: 9.4

Maybe Im doing the indexing wrong or I dont know. Really not familiar with indexing. Any help would be great. Thanks a lot!

Answer 1

Your problem is caused by you sorting on table_date rather than date(table_date) . This can be corrected by modyfing the query to:

SELECT DATE(table_date)
FROM sample
WHERE DATE(table_date) <= '8/30/2017'
ORDER BY DATE(table_date) DESC 
LIMIT 1