Re: Can I add Index to make a query faster which involves joins on unnest ?

[Rob Sargent <robjsargent@xxxxxxxxx>]

On Sep 13, 2018, at 12:17 PM, Arup Rakshit <ar@xxxxxxx> wrote:

The below query basically gives the result by maintaining the order of the sizes in the list.

explain analyze select

        "price_levels"."name",

        "price_levels"."size"

from

        "price_levels"

join unnest(array['M',

        'L',

        'XL',

        '2XL',

        '3XL',

        '4XL',

        '5XL',

        '6XL',

        'S']) with ordinality t(size,

        ord)

                using (size)

order by

        t.size

I have a Btree index on the size column.

Explain output is:

Merge Join  (cost=4.61..5165.38 rows=60000 width=46) (actual time=0.157..57.872 rows=60000 loops=1)

  Merge Cond: ((price_levels.size)::text = t.size)

  ->  Index Scan using price_levels_size_idx on price_levels  (cost=0.29..4111.05 rows=60000 width=14) (actual time=0.044..25.941 rows=60000 loops=1)

  ->  Sort  (cost=4.32..4.57 rows=100 width=32) (actual time=0.108..3.946 rows=53289 loops=1)

        Sort Key: t.size

        Sort Method: quicksort  Memory: 25kB

        ->  Function Scan on unnest t  (cost=0.00..1.00 rows=100 width=32) (actual time=0.030..0.033 rows=9 loops=1)

Planning time: 0.667 ms

Execution time: 62.846 ms

Thanks,

Arup Rakshit

ar@xxxxxxx

There are not value of size fit it to be a worthwhile key.