Re:Re: Re: Re: Different execution plan between PostgreSQL 8.4 and 12.11

Hi, Pavel

> The LIMIT clause changes total cost. This is a very aggressive clause. And

> although it is absolutely useless in this case, Postgres does not have any

> logic for removing it. Postgres doesn't try to fix developer's mistakes.

Sorry,I didn't understand what you mean.

Couldn't the LIMIT clause be used like the SQL statement below?

>> new=# explain analyze select 2 from analyze_word_reports where (cseid =

>> 94) limit 1;

This SQL statement is no problem under PostgreSQL 8.4, the index works well.

At 2022-10-11 12:13:47, "Pavel Stehule" <pavel(dot)stehule(at)gmail(dot)com> wrote:

út 11. 10. 2022 v 6:05 odesílatel gzh <gzhcoder(at)126(dot)com> napsal:

Hi, Pavel

Thank you for your reply.

> the LIMIT clause is in this case totally useless and messy, and maybe can

> negative impacts optimizer

Yes. After removing the LIMIT clause, the performance is improved.

The execution plan shows that the index worked.

We've noticed it, but I don't want to fix the problem by modifying the SQL until I find the cause.

The LIMIT clause changes total cost. This is a very aggressive clause. And although it is absolutely useless in this case, Postgres does not have any logic for removing it. Postgres doesn't try to fix developer's mistakes.

At 2022-10-11 11:32:48, "Pavel Stehule" <pavel(dot)stehule(at)gmail(dot)com> wrote:

út 11. 10. 2022 v 5:13 odesílatel gzh <gzhcoder(at)126(dot)com> napsal:

Hi, Tom
Thank you for your reply.

> When you're asking for help, please don't give us vague statements

> like "doesn't seem to work".

I understand.

> Did the plan (including rowcount

> estimates) change at all? To what? How far off is that rowcount

> estimate, anyway --- that is, how many rows actually have cseid = 94?

Please refer to the new execution plan (PostgreSQL 12.11) below.

new=# show enable_seqscan;

enable_seqscan

----------------

on

(1 行)

new=# select count(*) from analyze_word_reports;

count

----------

21331980

(1 行)

new=# select count(*) from analyze_word_reports where (cseid = 94);

count

---------

1287156

(1 行)

new=# explain analyze select count(2) from analyze_word_reports where (cseid = 94) limit 1;

QUERY PLAN

the LIMIT clause is in this case totally useless and messy, and maybe can negative impacts optimizer

Regards

Pavel

----------------------------------------------------------------------------------------------------------------------------------

--------------------------------------------------------------------------

Limit (cost=65184.06..65184.07 rows=1 width=8) (actual time=123.713..133.035 rows=1 loops=1)

-> Finalize Aggregate (cost=65184.06..65184.07 rows=1 width=8) (actual time=123.712..133.033 rows=1 loops=1)

-> Gather (cost=65183.85..65184.06 rows=2 width=8) (actual time=123.548..133.024 rows=3 loops=1)

Workers Planned: 2

Workers Launched: 2

-> Partial Aggregate (cost=64183.85..64183.86 rows=1 width=8) (actual time=119.495..119.496 rows=1 loops=3)

-> Parallel Index Only Scan using analyze_word_reports_index_cseid on analyze_word_reports (cost=0.56..6290

9.34 rows=509805 width=0) (actual time=0.031..98.706 rows=429052 loops=3)

Index Cond: (cseid = 94)

Heap Fetches: 1287156 Planning Time: 0.122 ms Execution Time: 133.069 ms

(11 行)

new=# explain analyze select 2 from analyze_word_reports where (cseid = 94) limit 1;

QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------

---

Limit (cost=0.00..0.43 rows=1 width=4) (actual time=2156.964..2156.966 rows=1 loops=1)

-> Seq Scan on analyze_word_reports (cost=0.00..528550.75 rows=1223533 width=4) (actual time=2156.962..2156.964 rows=1 loops=

1)

Filter: (cseid = 94)

Rows Removed by Filter: 18320180 Planning Time: 0.086 ms Execution Time: 2156.985 ms

(6 行)

> If the estimate is far off, then increasing the table's statistics

> target might help.

Thank you for your advice.

Please tell me how to set the table's statistics up to improve performance.

new=# select oid from pg_class where relname = 'analyze_word_reports';

oid

-------

16429

(1 行)

new=# select attrelid,attname,attstattarget from pg_attribute where attrelid=16429 and attname='cseid';

attrelid | attname | attstattarget

----------+---------+---------------

16429 | cseid | -1

(1 行)

> Another thing that would be worth checking is whether

> "set enable_seqscan = off" prods it to choose the plan you want.

> If not, then there's something else going on besides poor estimates.

"set enable_seqscan = off" works, and the performance is greatly improved, which is almost the same as PostgreSQL 8.4.

The enable_seqscan(PostgreSQL 8.4) is on, will this change have an unknown effect on other queries?

At 2022-10-10 10:45:54, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>gzh <gzhcoder(at)126(dot)com> writes:
>> I've run analyze(not vacuum analyze), but it doesn't seem to work.
>
>When you're asking for help, please don't give us vague statements
>like "doesn't seem to work". Did the plan (including rowcount
>estimates) change at all? To what? How far off is that rowcount
>estimate, anyway --- that is, how many rows actually have cseid = 94?
>
>If the estimate is far off, then increasing the table's statistics
>target might help.
>
>Another thing that would be worth checking is whether
>"set enable_seqscan = off" prods it to choose the plan you want.
>If not, then there's something else going on besides poor estimates.
>
> regards, tom lane