Re: Differents execution times with gin index, prepared statement and literals.

Hello,

Thanks a lot for your clear answer.

On 7/16/24 17:43, Pierrick Chovelon wrote:

...

Quite fast as well...

Have you got an idea on the initial issue ? Why when using a prepared
statement and a gin index the execution time "explode" ?
Something to do with the planner ? optimizer ?

(We executed the same test with a btree index and execution times are
the same in both cases).

The reason why the two queries end up with different plans is pretty
simple - the condition ends up matching different operators, because of
data type difference. In case of the prepared query, the (x <= 950000)
matches <=(bigint,bitint) operator, and thus it matches the index. But
that happens because the query is prepared with bigint parameter. For
the regular query, the 950000 literal gets treated as int, the condition
matches to <=(bigint,int) and that does not match the index - hence it's
treated as a filter, not an index condition.

If you cast the literal to bigint (by doing ::bigint) in the regular
query, we end it'll use the same same plan as the prepared query - but
that's the slower one, unfortunately :-(

postgres=# prepare stmt(int, text, int) as delete from  tmp_tk_test_index where sync_id <= $1 and line_id = $2 and chk_upgrade_index = $3;
PREPARE
postgres=# begin ;
BEGIN
postgres=*# explain (analyse) execute stmt(950000, 'the-test-value-fa529a621a15', 0);

                                                              QUERY PLAN                                               

-----------------------------------------------------------------------------------------------------------------------

 Delete on tmp_tk_test_index  (cost=21.36..25.38 rows=0 width=0) (actual time=0.148..0.149 rows=0 loops=1)
   ->  Bitmap Heap Scan on tmp_tk_test_index  (cost=21.36..25.38 rows=1 width=6) (actual time=0.146..0.147 rows=0 loops=1)
         Recheck Cond: ((line_id)::text = 'the-test-value-fa529a621a15'::text)
         Filter: ((sync_id <= 950000) AND (chk_upgrade_index = 0))
         Heap Blocks: exact=1
         ->  Bitmap Index Scan on idx_tmp_tk_test_index_1  (cost=0.00..21.36 rows=1 width=0) (actual time=0.099..0.099 rows=1 loops=1)
               Index Cond: ((line_id)::text = 'the-test-value-fa529a621a15'::text)
 Planning Time: 9.412 ms
 Execution Time: 1.570 ms
(9 rows)
postgres=*# rollback ;
ROLLBACK

So preparing a query with a data type different from the column (int (prepared statement) vs bigint (table)) is faster in our case :/
It doesn't sound obvious to me :)

Thanks again for your answer Tomas.

Which gets us to why that plan is slower, compared to the plan using
fewer conditions. I think the problem is that <= 950000 matches most of
the table, which means the GIN index will have to load and process a
pretty long TID list, which is clearly not cheap.

I don't think there's much you can do do - we don't consider this when
matching conditions to the index, we simply match as many conditions as
possible. And the GIN code is not smart enough to make judgements about
which columns to process first - it just goes column by column and
builds the bitmap, and building a bitmap on 95% of the table is costly.

If this is a systemic problem for most/all queries (i.e. it's enough to
have a condition on line_id), I believe the +0 trick is a good way to
make sure the condition is treated as a filter.


regards

-- 
Pierrick Chovelon
Consultant DBA PostgreSQL - Dalibo