On Thu, September 26, 2013 00:34, Erik Rijkers wrote:
> On Wed, September 25, 2013 22:34, Alvaro Herrera wrote:
>
>> [minmax-5.patch]
>
> I have the impression it's not quite working correctly.
>
> The attached program returns different results for different values of
> enable_bitmapscan (consistently).
>
> ( Btw, I had to make the max_locks_per_transaction higher for even
> not-so-large tables -- is that expected? For a 100M row
> table, max_locks_per_transaction=1024 was not enough; I set it to 2048.
> Might be worth some documentation, eventually. )
>
> From eyeballing the results it looks like the minmax result (i.e. the result
> set with enable_bitmapscan = 1) yields only
> the last part because the only 'last' rows seem to be present (see the values
> in column i in table tmm in the attached
> program).
Looking back at that, I realize I should have added a bit more detail on that
test.sh program and its output (attached on
previous mail).
test.sh creates a table tmm and a minmax index on that table:
testdb=# \d tmm
Table "public.tmm"
Column | Type | Modifiers
--------+---------+-----------
i | integer |
r | integer |
Indexes:
"tmm_minmax_idx" minmax (r)
The following shows the problem: the same search with minax index on versus
off gives different result sets:
testdb=# set enable_bitmapscan=0; select count(*) from tmm where r between
symmetric 19494484 and 145288238;
SET
Time: 0.473 ms
count
-------
1261
(1 row)
Time: 7.764 ms
testdb=# set enable_bitmapscan=1; select count(*) from tmm where r between
symmetric 19494484 and 145288238;
SET
Time: 0.471 ms
count
-------
3
(1 row)
Time: 1.014 ms
testdb=# set enable_bitmapscan =1; select * from tmm where r between symmetric
19494484 and 145288238;
SET
Time: 0.615 ms
i | r
------+-----------
9945 | 45405603
9951 | 102552485
9966 | 63763962
(3 rows)
Time: 0.984 ms
testdb=# set enable_bitmapscan=0; select * from ( select * from tmm where r
between symmetric 19494484 and 145288238 order
by i desc limit 10) f order by i ;
SET
Time: 0.470 ms
i | r
------+-----------
9852 | 114996906
9858 | 69907169
9875 | 43341583
9894 | 127862657
9895 | 44740033
9911 | 51797553
9916 | 58538774
9945 | 45405603
9951 | 102552485
9966 | 63763962
(10 rows)
Time: 8.704 ms
testdb=#
If enable_bitmapscan=1 (i.e. using the minmax index), then only some values are
retrieved (in this case 3 rows). It turns
out those are always the last N rows of the full resultset (i.e. with
enable_bitmapscan=0).
Erikjan Rijkers
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