Re: Minmax indexes

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