Re: Bug 4906 -- Left join of subselect incorrect

Hi all,

After running the attached setup.sql.gz SQL script on a PostgreSQL
8.4.0 database, the following two queries which should be logically
identical return different results. As far as I can tell from the
query analysis, the LEFT JOIN on query A is happening after
"ee.projectid = pc.projectid" is filtered; therefore the rows where
projectid is NULL are not visible in query A. The issue does not
occur in PostgreSQL 8.3.6.

My apologies for the large test setup; I attempted build up the same
test case, but was unable to reproduce the issue. I had to tear down
my database as much as I could while maintaining the issue.

Query A:
select *
FROM expense ex
JOIN expenseentry ee ON ex.id = ee.expenseid
LEFT JOIN (
SELECT projectclient.projectid, projectclient.clientid,
projectclient.billingpercentage
FROM projectclient
WHERE projectclient.projectid IN (
SELECT project.id
FROM project
WHERE project.clientbillingallocationmethod <> 2)
) pc ON ee.projectid = pc.projectid

Query B:
select *
FROM expense ex
JOIN expenseentry ee ON ex.id = ee.expenseid
LEFT JOIN (
SELECT projectclient.projectid, projectclient.clientid,
projectclient.billingpercentage
FROM projectclient
INNER JOIN project ON (projectclient.projectid = project.id)
WHERE project.clientbillingallocationmethod <> 2
) pc ON ee.projectid = pc.projectid

Mathieu Fenniak <mathieu(at)fenniak(dot)net> writes:
> After running the attached setup.sql.gz SQL script on a PostgreSQL
> 8.4.0 database, the following two queries which should be logically
> identical return different results. As far as I can tell from the
> query analysis, the LEFT JOIN on query A is happening after
> "ee.projectid = pc.projectid" is filtered; therefore the rows where
> projectid is NULL are not visible in query A.

Yeah, it seems to be confused about whether it can interchange the
order of the semijoin and left join. I'll take a look.

regards, tom lane

Mathieu Fenniak <mathieu(at)fenniak(dot)net> writes:
> After running the attached setup.sql.gz SQL script on a PostgreSQL
> 8.4.0 database, the following two queries which should be logically
> identical return different results. As far as I can tell from the
> query analysis, the LEFT JOIN on query A is happening after
> "ee.projectid = pc.projectid" is filtered; therefore the rows where
> projectid is NULL are not visible in query A.

This should fix it. Thanks for the report!

regards, tom lane

On 20-Jul-09, at 8:05 PM, Tom Lane wrote:
>
> This should fix it. Thanks for the report!
>
> regards, tom lane

Just tested this patch, and it works perfectly. Thank-you.

Mathieu