Re: pb with join plan

On Wed, Jun 21, 2023 at 12:10 PM Marc Millas <marc(dot)millas(at)mokadb(dot)com> wrote:

> Marc MILLAS
> On Wed, Jun 21, 2023 at 12:43 PM Tomas Vondra <
> tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>
>> On 6/21/23 00:26, Marc Millas wrote:
>> >
>> >
>> > On Tue, Jun 20, 2023 at 11:19 PM David Rowley <dgrowleyml(at)gmail(dot)com
>> > <mailto:dgrowleyml(at)gmail(dot)com>> wrote:
>> >
>> > On Wed, 21 Jun 2023 at 08:34, Marc Millas <marc(dot)millas(at)mokadb(dot)com
>> > <mailto:marc(dot)millas(at)mokadb(dot)com>> wrote:
>> > >
>> > > On Tue, Jun 20, 2023 at 10:14 PM David Rowley
>> > <dgrowleyml(at)gmail(dot)com <mailto:dgrowleyml(at)gmail(dot)com>> wrote:
>> > >>
>> > >> On Wed, 21 Jun 2023 at 07:42, Marc Millas <
>> marc(dot)millas(at)mokadb(dot)com
>> > <mailto:marc(dot)millas(at)mokadb(dot)com>> wrote:
>> > >> > But if I do the same with clause one OR clause 2, I have to
>> > kill the request after an hour, seeing the filesystem showing more
>> > than 140 Mb of increased usage.
>>
>> It's a bit weird the "victor" table is joined seemingly without any join
>> conditions, leading to a cross join (which massively inflates the cost
>> for joins above it). Maybe the anonymized plan mangles it somehow.
>>
>
> So I did try to simplify my pb.
> I create a table with the result of the first 3 joins.
> That table do have 15M lines. all tables have been vacuum analyze
>
> Now if I do an explain analyze of a simple join between that table and my
> original table 4
> using a simple = clause, I get a result in one second (around). and the
> planner guesses for rows seems in line with the observed values .
> if I use a substr(table1.a)= table2.b, the explain analyze get a result in
> 21 seconds and the planner estimates a 65M rows result set while the
> observed is 330 k rows
> so here its 20 times slower and the discrepency between planner rows guess
> and reality is a 200 ratio.
>
> Now, if I try an explain analyze with join on a=b or substr(c)=d or e=f
> then... I kill the query after a quarter an hour without any answer.
> if I try to just explain the query, the planner rows guess becomes more
> than 2 Billions....
> the extremely simple query and plan are here, without automatic obfuscation
> https://explain.depesz.com/s/b8Ll
>

First, I am not sure why you cannot send us the explain analyze. But
moving on...

substr() is a function that mutilates a value such that the index becomes
useless...
If you are looking for the LEFT() of the value, then an INDEX can be used.
I have COLLATION "C" and when I query:
WHERE fld like fld_b||"%"

The optimizer constructs a query that uses the index on "fld"...
But when I try:

WHERE fld like CONCAT_WS("", fld_b,"%")
It doesn't use the index version. (because the function call is too
complicated to see through)

When using functions in where clauses, indexes either have to be made on
those functions, or often times the index cannot be used.

BTW, I noted the COLLATION. That turned out to be important, because my
first DB test did NOT use that collation, and the result
of the LIKE was the non-indexed version...

I hope you find something useful in here.

Also, WHERE fld <> 72... (unless you have a heavily skewed set of
statistics, I read that as. SCAN everything, and check later,
because this should filter very few rows), whereas fld = 72 will be
blazingly fast.

Kirk