Someone asked a hypothetical question about how to retrieve all records of a
table twice in SQL. It got me thinking about whether there was a way to do
this efficiently.
"Obviously" if you do it using the UNION ALL approach postgres isn't going to
do two separate scans, doing it otherwise would be quite hard.
However using the join approach it seems postgres ought to be able to do a
single sequential scan and return every tuple it finds twice. It doesn't do
this:
slo=> explain analyze select * from region, (select 1 union all select 2) as x;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
Nested Loop (cost=0.00..11162.00 rows=5534 width=108) (actual time=0.13..541.19
rows=5534 loops=1)
-> Subquery Scan x (cost=0.00..2.00 rows=2 width=0) (actual time=0.03..0.08
rows=2 loops=1)
-> Append (cost=0.00..2.00 rows=2 width=0) (actual time=0.02..0.05 rows=2
loops=1)
-> Subquery Scan "*SELECT* 1" (cost=0.00..1.00 rows=1 width=0)
(actual time=0.01..0.02 rows=1 loops=1)
-> Result (cost=0.00..1.00 rows=1 width=0) (actual
time=0.01..0.01 rows=1 loops=1)
-> Subquery Scan "*SELECT* 2" (cost=0.00..1.00 rows=1 width=0)
(actual time=0.01..0.02 rows=1 loops=1)
-> Result (cost=0.00..1.00 rows=1 width=0) (actual
time=0.01..0.01 rows=1 loops=1)
-> Seq Scan on region (cost=0.00..2813.00 rows=2767 width=104) (actual
time=0.03..123.44 rows=2767 loops=2)
Total runtime: 566.24 msec
(9 rows)
Wouldn't it be faster to drive the nested loop the other way around?
(I'm also a bit puzzled why the optimizer is calculating that 2,813 * 2 = 5,534)
This is tested on 7.3. I haven't tried CVS yet.
--
greg
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