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

On 14 Aug 2017, at 01:15, Melvin Davidson <melvin6925@gmail.com> wrote:

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.

Is that true even if that index is a descending one?

As far as "Index only scan" , since the table only has 2003 rows, the optimizer has determined it is faster just to

load all the rows into memory and then filter. If you really want to force an index scan, then you would have to do

SET enable_seqscan = off; Before doing the query, however you are just shooting yourself in the foot by doing that

as it will make the query slower.

I will try to load up more dummy rows to overflow the work_mem and observe results.

Sorry, my question was misleading. I do not want to use "set enable_seqscan = off" I want to be sure that when necessary (record count increases) relevant index(es) will be used.

Obviously I still can't read query plan as I did not understand that operation is still in memory.

Just for complete information, this is 64bit PostgreSQL 9.6.4 on Windows 10, EDB binaries.

Thanks.

Ertan Küçükoğlu