Hi,
I have reanalysed the code of function _bt_first. I notice that using a
multi-attribute index
if we can't identify the starting boundaries and the following attributes
markded not required ,
that means we need start at first or last page in the index to examine every
tuple to satisfy the
qual or not, in the meantime the scan will be stopped while the first attribute
evaluated failed.
For instance:
create table c_s( x int, y int);
insert into c_s select generate_series(1, 2), generate_series(1, 2);
create index my_c_s_idx on c_s using btree(x,y);
explain (analyze, buffers) select * from c_s where x >4000 and y >10 and y
<10 order by x desc;
Index Only Scan Backward using my_c_s_idx on c_s (cost=0.29..384.31 rows=1
width=8) (actual time=1.302..1.304 rows=0 loops=1)
Index Cond: ((x > 4000) AND (y > 10) AND (y < 10))
Heap Fetches: 0
Buffers: shared read=46
Planning:
Buffers: shared hit=51 read=15
Planning Time: 1.311 ms
Execution Time: 1.435 ms
(8 rows)
The instance is a little different for description above due to the implies NOT
NULL Scankey,
but it has no effect on the whole situation.
What's more, if i change the number 4000 to 1000.
-
Sort (cost=441.01..441.01 rows=1 width=8) (actual time=2.974..2.975 rows=0
loops=1)
Sort Key: x DESC
Sort Method: quicksort Memory: 25kB
Buffers: shared hit=89
-> Seq Scan on c_s (cost=0.00..441.00 rows=1 width=8) (actual
time=2.971..2.971 rows=0 loops=1)
Filter: ((x > 1000) AND (y > 10) AND (y < 10))
Rows Removed by Filter: 2
Buffers: shared hit=89
Planning:
Buffers: shared hit=2
Planning Time: 0.113 ms
Execution Time: 2.990 ms
(12 rows)
The planner choosed the Seq Scan, and the executor have done the unnecessary
jobs 2 times.
Let's don't confine to the plain attributes or row comparison and Seq Scan or
Index Scan .
We can pretend row-comparison as multi-attributes comparison. The qual is
implicit-AND format,
that means once one attribute is self-contradictory, we can abandon the qual
immediately.
Maybe we can do more efficient jobs during planning time. Like at the phase of
function deconstruct_recurse
invoked, we can identify qual that is self-contradictory then flag it. With
this information we know who is
a dummy relation named arbitrarily.
For instance:
explain (analyze, buffers) select * from c_s a , c_s b where a.y >10 and a.y<10
and a.x=b.x;
QUERY PLAN
---
Nested Loop (cost=0.29..393.31 rows=1 width=16) (actual time=1.858..1.858
rows=0 loops=1)
Buffers: shared hit=89
-> Seq Scan on c_s a (cost=0.00..389.00 rows=1 width=8) (actual
time=1.857..1.857 rows=0 loops=1)
Filter: ((y > 10) AND (y < 10))
Rows Removed by Filter: 2
Buffers: shared hit=89
-> Index Only Scan using my_c_s_idx on c_s b (cost=0.29..4.30 rows=1
width=8) (never executed)
Index Cond: (x = a.x)
Heap Fetches: 0
Planning:
Buffers: shared hit=12
Planning Time: 0.236 ms
Execution Time: 1.880 ms
(13 rows)
After deconstruct_recurse invoked, qual(a.y >10 and a.y<10) was distributed to
relation "a" and relation "a" is
a dummy relation. When "a" and "b" make inner join, "a" will supply nothing.
That means the inner-join rel is
a dummy relation too. If there is a outer join above the inner join and some
one who can commute with it,
we can do more efficient jobs and so on.
It also has benefit on path-competing phase with this feature due to it doesn't
cost anything.
It's need to discuss the idea whether is feasible or not and it will takes a
lot of effort to complete this feature.
Anyway we can fix these issues what we had encountered first.
bigbro...@hotmail.com
From: Peter Geoghegan
Date: 2024-08-30 22:32
To: b ro
CC: pgsql-hackers
Subject: Re: bt Scankey in another contradictory case
On Fri, Aug 30, 2024 at 7:36 AM b ro wrote:
> this is the patch attachment.
We discussed this recently:
https://www.postgresql.org/message-id/80384.1715458896%40sss.pgh.pa.us
I think that we should do this.
It doesn't make a huge difference in practice, because we'll still end
the scan once the leaf level is reached. But it matters more when
array keys are involved, where there might be more than one descent to
the leaf level. Plus we might as well just be thorough about this
stuff.
--
Peter Geoghegan
