Re: pgsql 10.23 , different systems, same table , same plan, different Buffers: shared hit

Στις 15/9/23 18:23, ο/η Tom Lane έγραψε:
> Achilleas Mantzios - cloud<a(dot)mantzios(at)cloud(dot)gatewaynet(dot)com> writes:
>> *FreeBSD*
>>
>> -> Index Only Scan using mail_vessel_addressbook_address_regex_idx
>> on mail_vessel_addressbook (cost=0.42..2912.06 rows=620 width=32)
>> (actual time=96.704..96.705 rows=1 loops=1)
>> Filter: ('foo(at)bar(dot)com'::text ~* address_regex)
>> Rows Removed by Filter: 14738
>> Heap Fetches: 0
>> Buffers: shared hit=71
>>
>> *Linux*
>>
>> -> Index Only Scan using mail_vessel_addressbook_address_regex_idx
>> on mail_vessel_addressbook (cost=0.42..2913.04 rows=620 width=32)
>> (actual time=1768.724..1768.725 rows=1 loops=1)
>> Filter: ('foo(at)bar(dot)com'::text ~* address_regex)
>> Rows Removed by Filter: 97781
>> Heap Fetches: 0
>> Buffers: shared hit=530
>> The file in FreeBSD came by pg_dump from the linux system, I am puzzled
>> why this huge difference in Buffers: shared hit.
> The "rows removed" value is also quite a bit different, so it's not
> just a matter of buffer touches --- there's evidently some real difference
> in how much of the index is being scanned. I speculate that you are
> using different collations on the two systems, and FreeBSD's collation
> happens to place the first matching row earlier in the index.

Thank you, I see that both systems use en_US.UTF-8 as lc_collate and
lc_ctype, and that in both systems :

dynacom=# \dOS+
                               List of collations
  Schema   |  Name   | Collate | Ctype | Provider |         Description
------------+---------+---------+-------+----------+------------------------------

pg_catalog | C       | C       | C     | libc     | standard C collation
pg_catalog | POSIX   | POSIX   | POSIX | libc     | standard POSIX
collation
pg_catalog | default |         |       | default  | database's default
collation
(3 rows)

dynacom=# \l
                                  List of databases
  Name    |  Owner   | Encoding  |   Collate   |    Ctype    |   Access
privileges
-----------+----------+-----------+-------------+-------------+------------------------

dynacom   | postgres | SQL_ASCII | en_US.UTF-8 | en_US.UTF-8 |

the below seems ok

FreeBSD :

postgres(at)[local]/dynacom=# select * from (values
('a'),('Z'),('_'),('.'),('0')) as qry order by column1::text;
column1
---------
_
.
0
a
Z
(5 rows)

Linux:

dynacom=# select * from (values ('a'),('Z'),('_'),('.'),('0')) as qry
order by column1::text;
column1
---------
_
.
0
a
Z
(5 rows)

dynacom=#

but :

Freebsd :

postgres(at)[local]/dynacom=# select distinct address_regex from
mail_vessel_addressbook order by address_regex::text ASC limit 5;
                     address_regex
----------------------------------------------------------
_cmo.ship.inf@<hide>.<hid>
_EMD_REEFER(at)hide>.<hid>
_OfficeHayPoint(at)hide>.<hid>
_Sabtank_PCQ1_All_SSVSSouth_area(at)hide>.<hid>
_Sabtank_PCQ1_Lead_OperatorsSouth_area(at)hide>.<hid>
(5 rows)

While in Linux :

dynacom=# select distinct address_regex from mail_vessel_addressbook
order by address_regex::text ASC limit 5;
          address_regex
-----------------------------------
0033240902573@<hidden>.<hid>
0033442057364@<hidden>.<hid>
0072usl@<hidden>.<hid>

0081354426912@<hidden>.<hid>
00862163602861@<hidden>.<hid>
(5 rows)

somethings does not seem right.

>
> regards, tom lane

--
Achilleas Mantzios
IT DEV - HEAD
IT DEPT
Dynacom Tankers Mgmt