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FULL JOIN and mergjoinable conditions...

Today I got the error:

ERROR: FULL JOIN is only supported with mergejoinable join conditions

Which is really annoying since a full join is exactly what I wanted. I
guess the alternative is to do a left join and a right join and merge
them? Is it just that no-one has come up with a way to code this
efficiently?

Maybe someone has a better way to express this. The problem is I have
two tables with ranges and I wanted to generate a result with the
overlaps and blanks where there are things missed. For example:

Table A Table B
Tag Start End Tag Start End
A 1 2 A 2 7
B 6 9 B 9 9
C 10 12 C 13 15

So the query looks like:

SELECT * from A full outer join B on (a.end >= b.start and b.end >= a.start)

The result would be something like:

A 1 2 A 2 7
B 6 9 A 2 7
B 6 9 B 9 9
C 10 12 \N \N \N
\N \N \N C 13 15

Any ideas?
--
Martijn van Oosterhout <kl*****@svana.org> http://svana.org/kleptog/
Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
tool for doing 5% of the work and then sitting around waiting for someone
else to do the other 95% so you can sue them.


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Nov 23 '05 #1
1 3480
Martijn van Oosterhout <kl*****@svana.org> writes:
Today I got the error:
ERROR: FULL JOIN is only supported with mergejoinable join conditions
Which is really annoying since a full join is exactly what I wanted. I
guess the alternative is to do a left join and a right join and merge
them? Is it just that no-one has come up with a way to code this
efficiently?


How would you do it? It seems fairly impractical with an underlying
nestloop join --- you'd need persistent state for *every* row of the
inner relation to show whether any outer row had matched it.

You could imagine doing it with a hash join (mark every hash table entry
when it gets visited by an outer-row hash probe, then traverse the hash
table at the end to emit unvisited rows). But a quick look into
pg_operator convinces me that this would be pointless to implement,
because we have no interesting datatypes that support hash join but not
mergejoin. And hashjoins are only practical with relatively-small inner
relations anyway. Not to mention that hashjoin isn't any more amenable
to inequality join conditions than mergejoin is...

regards, tom lane

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Nov 23 '05 #2

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