Den 23.01.2017 13:05, skrev Kjell Rilbe
[email protected] [firebird-support]:
Hi,
I just needed to do a full outer join between two very simple tables G
and N, each with a single-column PK of identical type char(12). I
noticed that the plan produces was:
PLAN JOIN (N NATURAL, G NATURAL)
I stopped the query after ~5 minutes. Then I thought I can do this
calculation in two steps, so I did a left outer join from G to N
first, then a left outer join from N to G (equivalent to a right outer
join from G to N). These queries were executed with these plans:
PLAN JOIN (G NATURAL, N INDEX (PK_N))
PLAN JOIN (N NATURAL, G INDEX (PK_G))
They both executed in less than half a minute each.
Now, I was wondering why the full outer join can't do a left join
using the right table's PK and then a right join using the other
table's PK. This is essentially what I did with the latter two queries.
The full join operation seems to do something that's a lot less
efficient (whatever it is that it does).
I'm still on FB 2.5.2, so... has this been improved in 3.0?
Thanks,
Kjell
Not quite answering your question, Kjell, but I do remember having
observed that some queries with outer joins take too long and that I
concluded that Firebird is great with inner JOINs, but performancewise
rather poor with some outer JOINs. I think I once solved one such case
using a CTE (at least I attempted using a CTE. I think, but am not 100%
certain it worked. However, my query involved more than two tables, as
indicated by M in square brackets below):
WITH TMP(PK) as
(SELECT PK FROM G
UNION
SELECT PK FROM N)
SELECT ...
FROM TMP
LEFT JOIN G ON TMP.PK = G.PK
LEFT JOIN N ON TMP.PK = N.PK
[ JOIN M ON M.Field = COALESCE(G.Field, N.Field) ]
Good to read you found a similar way to solve your particular problem.
Set
