Re: BUG #16504: Wrapping query in EXISTS() causes sequential scans of tables

PG Bug reporting form <noreply@postgresql.org> writes:
> In this case, I have a query that selects from a table, joins a table, joins
> that same table, joins the first table, performs a NOT
> firsttable.value=lasttable.value to make sure that I'm not joining an item
> with itself. There are indexes for the conditions in each join. When I run
> this as a select query, the EXPLAIN output uses index scans the whole way
> through. When I SELECT EXISTS(thatquery) FROM firsttable WHERE firsttable.id
> IN (1,2,3), the EXPLAIN output switches to using sequential scans. This
> destroys performance on this 3.8m row table.

Are you claiming that 9.6 did better?  I'm not aware that we changed
anything significant about how a sub-select in a query's select list
works.

The table/view definitions alone are not enough to investigate this,
since it's unlikely that we'd get the same plans on an empty table
as a populated one.  Please see if you can make a self-contained test
case with some dummy data that reproduces the problem (ie better
plan on 9.6 than later).

            regards, tom lane