Re: [GENERAL] Help on Index only scan - Mailing list pgsql-general

On Sun, Aug 13, 2017 at 5:59 PM, Ertan Küçükoğlu <ertan.kucukoglu@1nar.com.tr> wrote:

Hello,

My table details:
robox=# \dS+ updates
                                                   Table "public.updates"
    Column     |  Type   |                         Modifiers
| Storage  | Stats target | Description
---------------+---------+--------------------------------------------------
---------+----------+--------------+-------------
 autoinc       | integer | not null default
nextval('updates_autoinc_seq'::regclass) | plain    |              |
 filename      | text    |
| extended |              |
 dateofrelease | date    |
| plain    |              |
 fileversion   | text    |
| extended |              |
 afile         | text    |
| extended |              |
 filehash      | text    |
| extended |              |
 active        | boolean |
| plain    |              |
Indexes:
    "updates_pkey" PRIMARY KEY, btree (autoinc)
    "update_filename" btree (filename)
    "updates_autoinc" btree (autoinc DESC)
    "updates_dateofrelease" btree (dateofrelease)
    "updates_filename_dateofrelease" btree (filename, dateofrelease)


robox=# select count(autoinc) from updates;
 count
-------
  2003
(1 row)

robox=# select autoinc, filename, fileversion from updates limit 10;
 autoinc |             filename             | fileversion
---------+----------------------------------+-------------
      18 | Robox.exe                        | 1.0.1.218
      19 | Robox.exe                        | 1.0.1.220
      20 | Robox.exe                        | 1.0.1.220
      21 | 8423bfc5a669864f9b66b6b15ce908b9 | 1.1.1.1
      22 | 4fdabb0c7adbc5a89fbe679ce76ccef9 | 1.1.1.1
      23 | f469d77bfa86c8917c7846c0f871137c | 1.1.1.1
      24 | bc10af4c8789718a9ca6565ea14cb17d | 1.1.1.1
      25 | d9f87ee46cdb41cd15c2f71ed599faf9 | 1.1.1.1
      26 | 6f7428a5364aae1d5914a66cba3e6f3b | 1.1.1.1
      27 | 66ec4cdb8d64ca1414f75c1fb9eaa518 | 1.1.1.1
(10 rows)

I want to have an index only scan for my below query:
select autoinc, fileversion from updates where filename = 'Robox.exe' order
by autoinc desc;

I simply could not understand planner and cannot provide right index for it.
Below index names "update_filename" and "updates_autoinc" are added just for
the query that I would like to have a index only scan plan. I also failed
with following indexes
"autoinc desc, filename, fileversion"
"autoinc desc, filename"

First 3 rows in above select results are actual data. You will find that I
have inserted about 2000 rows of dummy data to have somewhat meaningful plan
for the query.

Current planner result:
robox=# vacuum full;
VACUUM
robox=# explain analyze
robox-# select autoinc, fileversion
robox-# from updates
robox-# where filename = 'Robox.exe'
robox-# order by autoinc desc;
                                                          QUERY PLAN
----------------------------------------------------------------------------
--------------------------------------------------
 Sort  (cost=12.79..12.79 rows=3 width=12) (actual time=0.047..0.047 rows=3
loops=1)
   Sort Key: autoinc DESC
   Sort Method: quicksort  Memory: 25kB
   ->  Bitmap Heap Scan on updates  (cost=4.30..12.76 rows=3 width=12)
(actual time=0.040..0.040 rows=3 loops=1)
         Recheck Cond: (filename = 'Robox.exe'::text)
         Heap Blocks: exact=1
         ->  Bitmap Index Scan on update_filename  (cost=0.00..4.30 rows=3
width=0) (actual time=0.035..0.035 rows=3 loops=1)
               Index Cond: (filename = 'Robox.exe'::text)
 Planning time: 1.873 ms
 Execution time: 0.076 ms
(10 rows)


I appreciate any help on having right index(es) as I simply failed myself.

Regards,
Ertan Küçükoğlu

First, you do not need index "updates_autoinc", since autoinc is the Primary Key, you are just duplicating the index.