Hello,
Em 19/01/2017 11:04, Clailson escreveu:
Hi Phillip.
Not sure if it is all that common. Curious what if you put b.b IS NOT
NULL in the WHERE statement?
It's the question. In the company I work with, one of my clients asked
me: "Why PostgreSQL does not remove rows with null in column b (table
b), before joining, since these rows have no corresponding in table
a?" I gave the suggestion to put the IS NOT NULL in the WHERE
statement, but HE can't modify the query in the application.
I did the tests with Oracle and it uses a predicate in the query plan,
removing the lines where b.b is null. In Oracle, it´s the same plan,
with and without IS NOT NULL in the WHERE statement.
Being the client in question, I would like to make a little remark: What
we thought could be optimized here at first is on the row estimate of
the index scan; which could take null_frac into account. To put things
into perspective, our similar case in production has a table with 6
million lines where only 9.5k aren´t null for the join field, an the
over-estimation is throwing away good plans (like ~150ms execution time)
in favor of pretty bad ones (~80s execution time).
We´ve asked application people to put the where not null workaround,
which works great, and are waiting on an answer, but I believe getting
better estimates without that would be great if possible.
On 19/01/2017 09:34, Phillip Couto wrote:
NULL is still a value that may be paired with a NULL in a.a
Is that so? I would believe you would never get a match, as NULL <> NULL
On Jan 19, 2017, at 05:08, Clailson <clailson....@gmail.com
<mailto:clailson....@gmail.com>> wrote:
Hi,
Is there something in the roadmap to optimize the inner join?
I've this situation above. Table b has 400 rows with null in the
column b.
explain analyze select * from a inner join b on (b.b = a.a);
"Merge Join (cost=0.55..65.30 rows=599 width=16) (actual time=0.030..1.173 rows=599
loops=1)"
" Merge Cond: (a.a = b.b)"
" -> Index Scan using a_pkey on a (cost=0.28..35.27 rows=1000 width=8) (actual
time=0.014..0.364 rows=1000 loops=1)"
" -> Index Scan using in01 on b (cost=0.28..33.27 rows=1000 width=8) (actual
time=0.012..0.249 rows=600 loops=1)"
"Total runtime: 1.248 ms"
My question is: Why the planner isn't removing the null rows during
the scan of table b?
--
Clailson Soares Dinízio de Almeida
Regards,
Gustavo R. Montesino
Tribunal Regional do Trabalho da 2a Região
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