how to implement selectivity injection in postgresql

On 13-08-2014 13:33, Rajmohan C wrote:

SELECT c1, c2, c3, FROM T1, T2, T3
WHERE T1.x = T2.x AND
T2.y=T3.y AND
T1.x >= ? selectivity 0.00001 AND
T2.y > ? selectivity 0.5 AND
T3.z = ? selectivity 0.2 AND
T3.w = ?

I need to implement Selectivity injection as shown in above query in
PostgreSQL by which we can inject selectivity of each predicate or at least
selectivity at relation level directly as part of query. Is there any
on-going work on this front? If there is no ongoing work on this, How
should I start implementing this feature?

Do you want to force a selectivity? Why don't you let the optimizer do
it for you? Trust me it can do it better than you. If you want to force
those selectivities for an academic exercise, that information belongs
to catalog or could be SET before query starts.

Start reading backend/optimizer/README.

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PostgreSQL: Consultoria, Desenvolvimento, Suporte 24x7 e Treinamento

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On 13-08-2014 15:28, Rajmohan C wrote:

Yeah. I have to do it for my academic research. Is it available in
catalogs? It is to be computed at run time from the predicates in the query
right?

The selectivity information is available at runtime. See
backend/optimizer/path/costsize.c.

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On Wed, Aug 13, 2014 at 9:33 AM, Rajmohan C <csrajmohan@gmail.com> wrote:

SELECT c1, c2, c3, FROM T1, T2, T3
WHERE T1.x = T2.x AND
T2.y=T3.y AND
T1.x >= ? selectivity 0.00001 AND
T2.y > ? selectivity 0.5 AND
T3.z = ? selectivity 0.2 AND
T3.w = ?

I need to implement Selectivity injection as shown in above query in
PostgreSQL by which we can inject selectivity of each predicate or at least
selectivity at relation level directly as part of query. Is there any
on-going work on this front? If there is no ongoing work on this, How
should I start implementing this feature?

My plan was to create a boolean operator which always returns true, but
estimates its own selectivity as 0.001 (or better yet, parameterize that
selectivity estimate, if that is possible) which can be inserted into the
place where lower selectivity estimate is needed with an "AND".

And another one that always returns false, but has a selectivity estimate
near 1, for use in OR conditions when the opposite change is needed.

I think that will be much easier to do than to extent the grammar. And
probably more acceptable to the core team.

I think this could be done simply in an extension module without even
needing to change the core code, but I never got around to investigating
exactly how.

Cheers,

Jeff

Jeff Janes <jeff.janes@gmail.com> writes:

On Wed, Aug 13, 2014 at 9:33 AM, Rajmohan C <csrajmohan@gmail.com> wrote:

I need to implement Selectivity injection as shown in above query in
PostgreSQL by which we can inject selectivity of each predicate or at least
selectivity at relation level directly as part of query.

My plan was to create a boolean operator which always returns true, but
estimates its own selectivity as 0.001 (or better yet, parameterize that
selectivity estimate, if that is possible) which can be inserted into the
place where lower selectivity estimate is needed with an "AND".

That doesn't seem especially helpful/convenient, especially not if you're
trying to affect the estimation of a join clause. The last discussion
I remember on this subject was to invent a special dummy function that
would be understood by the planner and would work sort of like
__builtin_expect() in gcc:

selectivity(condition bool, probability float8) returns bool

Semantically the function would just return its first argument (and
the function itself would disappear at runtime) but the planner would
take the value of the second argument as a selectivity estimate overriding
whatever it might've otherwise deduced about the "condition". So
you'd use it like

SELECT ... WHERE selectivity(id = 42, 0.0001)

and get functionally the same results as for

SELECT ... WHERE id = 42

but with a different selectivity estimate for that WHERE condition.

regards, tom lane

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