Re: Why query plan is different?

2016-10-10 23:17 GMT+02:00 Andrzej Zawadzki <zawadaa(at)gmail(dot)com>:

> On 10.10.2016 17:31, Andrzej Zawadzki wrote:
>
> Hi,
> Today, I noticed strange situation:
>
> The same query run on different servers has very different plan:
>
> Q: SELECT b.* FROM kredytob b WHERE pesel = '22222222222' ORDER BY b.id
> DESC LIMIT 1
>
> Slow plan:
>
> "Limit (cost=0.43..28712.33 rows=1 width=4) (actual
> time=2574.041..2574.044 rows=1 loops=1)"
> " Output: id"
> " Buffers: shared hit=316132 read=110001"
> " -> Index Scan Backward using kredytob_pkey on public.kredytob b
> (cost=0.43..3244444.80 rows=113 width=4) (actual time=2574.034..2574.034
> rows=1 loops=1)"
> " Output: id"
> " Filter: (b.pesel = '22222222222'::bpchar)"
> " Rows Removed by Filter: 433609"
> " Buffers: shared hit=316132 read=110001"
> "Planning time: 0.414 ms"
> "Execution time: 2574.139 ms"
>
>
> Fast plan:
> "Limit (cost=115240.66..115240.66 rows=1 width=4) (actual
> time=463.275..463.276 rows=1 loops=1)"
> " Output: id"
> " Buffers: shared hit=14661 read=4576"
> " -> Sort (cost=115240.66..115240.94 rows=112 width=4) (actual
> time=463.271..463.271 rows=1 loops=1)"
> " Output: id"
> " Sort Key: b.id DESC"
> " Sort Method: top-N heapsort Memory: 25kB"
> " Buffers: shared hit=14661 read=4576"
> " -> Index Scan using kredytob_pesel_typkred_opclass_idx on
> public.kredytob b (cost=0.43..115240.10 rows=112 width=4) (actual
> time=311.347..463.183 rows=5 loops=1)"
> " Output: id"
> " Index Cond: (b.pesel = '22222222222'::bpchar)"
> " Buffers: shared hit=14661 read=4576"
> "Planning time: 0.383 ms"
> "Execution time: 463.324 ms"
>
> Data is almost equal - "slow" has a few more rows in table. ("Fast" is a
> copy from 1 am today).
> Why runtime is slower?
>
>
> I made another INDEX, without opclass:
>
> CREATE INDEX kredytob_pesel_typkred_idx
> ON public.kredytob
> USING btree
> (pesel COLLATE pg_catalog."default", typkred);
>
> after that: analyze kredytob;
>
> And now:
> "Limit (cost=333.31..333.31 rows=1 width=4) (actual time=0.100..0.102
> rows=1 loops=1)"
> " Output: id"
> " Buffers: shared hit=8"
> " -> Sort (cost=333.31..333.59 rows=114 width=4) (actual
> time=0.095..0.095 rows=1 loops=1)"
> " Output: id"
> " Sort Key: b.id DESC"
> " Sort Method: top-N heapsort Memory: 25kB"
> " Buffers: shared hit=8"
> " -> Index Scan using kredytob_pesel_typkred_idx on
> public.kredytob b (cost=0.43..332.74 rows=114 width=4) (actual
> time=0.046..0.065 rows=5 loops=1)"
> " Output: id"
> " Index Cond: (b.pesel = '22222222222'::bpchar)"
> " Buffers: shared hit=8"
> "Planning time: 0.438 ms"
> "Execution time: 0.154 ms"
>
> So, what is a reason that "SLOW" server doesn't like opclass index?
>

what is default locales?

Pavel

>
> --
> Andrzej
>