Re: [PATCHES] WIP Join Removal

On Tue, 2008-09-02 at 12:05 +0100, Gregory Stark wrote:

> I wonder if it would be more worthwhile to remove them and have a subsequent
> phase where we look for possible joins to *add*. So even if the user writes
> "select * from invoices where customer_id=?" the planner might be able to
> discover that it can find those records quicker by scanning customer, finding
> the matching <company_id,customer_id> and then using an index to look them up
> in invoices.

This seems a less useful idea now just simply because it is such a
special case.

We would need to have a case where we have a table A that does not have
an index on a specific column, yet table B does have an index on the
specific column. But also when A references B as a foreign key and where
the column is a subset of the columns of the primary key of B.

That means only queries like

select ...
from a
where a.col2 = x;

can be transformed into

select ...
from a join b on (foreign key cols)
where a.col2 = x;

and then because a.col2 is a subset of foreign key columns we can infer
that b.col2 = x.

So the pre-conditions for this to be useful are:
* constraint on subset of a FK
* subset of FK is indexed on B
* subset of FK is not indexed on A

Which doesn't seem that likely to occur.

Thanks both to Heikki and Greg for good, fast input on this patch.
Nothing more needed now while I rework patch.

--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support

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