Re: Slow 3 Table Join with v bad row estimate

Yeah, the planner is not nearly smart enough to draw any useful
conclusions about the selectivity of that clause from standard statistics.
What you might try doing is creating functional indexes on the two
subexpressions:

create index on branch_purchase_order ((branch_code || po_number));
create index on stocksales_ib (replace(order_no,' ',''));

(actually it looks like you've already got the latter one) and then
re-ANALYZING.  I'm not necessarily expecting that the planner will
actually choose to use these indexes in its plan; but their existence
will prompt ANALYZE to gather stats about the expression results,
and that should at least let the planner draw more-accurate conclusions
about the selectivity of the equality constraint.

                        regards, tom lane