Re: BUG #16183: PREPARED STATEMENT slowed down by jit

The queries are very simple, like:

PREPARE mark_ways_by_node(" POSTGRES_OSMID_TYPE ") AS select id from planet_osm_ways WHERE nodes && ARRAY[$1];

They are all located here in osm2pgsql source code:

https://github.com/openstreetmap/osm2pgsql/blob/master/src/middle-pgsql.cpp#L786

Here are the EXPLAIN/ANALYZE without jit and with jit:

osm=# explain analyze execute mark_ways_by_node(1836953770);
                                                              QUERY PLAN                                                             
--------------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on planet_osm_ways  (cost=2468.37..305182.32 rows=301467 width=8) (actual time=0.039..0.042 rows=2 loops=1)
   Recheck Cond: (nodes && '{1836953770}'::bigint[])
   Heap Blocks: exact=2
   ->  Bitmap Index Scan on planet_osm_ways_nodes  (cost=0.00..2393.00 rows=301467 width=0) (actual time=0.029..0.029 rows=2 loops=1)
         Index Cond: (nodes && '{1836953770}'::bigint[])
 Planning Time: 0.171 ms
 Execution Time: 0.077 ms
(7 rows)


osm=# PREPARE mark_ways_by_node(bigint) AS select id from planet_osm_ways WHERE nodes && ARRAY[$1];
PREPARE
osm=# explain analyze execute mark_ways_by_node(1836953770);
                                                              QUERY PLAN                                                             
--------------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on planet_osm_ways  (cost=2468.37..305182.32 rows=301467 width=8) (actual time=5.775..5.905 rows=2 loops=1)
   Recheck Cond: (nodes && '{1836953770}'::bigint[])
   Heap Blocks: exact=2
   ->  Bitmap Index Scan on planet_osm_ways_nodes  (cost=0.00..2393.00 rows=301467 width=0) (actual time=0.512..0.512 rows=2 loops=1)
         Index Cond: (nodes && '{1836953770}'::bigint[])
 Planning Time: 3.667 ms
 JIT:
   Functions: 4
   Options: Inlining false, Optimization false, Expressions true, Deforming true
   Timing: Generation 0.466 ms, Inlining 0.000 ms, Optimization 0.354 ms, Emission 4.634 ms, Total 5.454 ms
 Execution Time: 30.393 ms
(11 rows)

The update tool (osm2pgsql) makes heavy use of PREPARED statements. Could
there be a bug in jit+PREPARED causing the statement to be recompiled on
each execute ?

I don't think that that is a bug, rather it is just how JIT works at the moment (that it is recompiled for each execution).  That might be improved in a future version.

For the slow down to be 10x though does seem rather extreme.  Could you isolate the query and post an execution with "EXPLAIN (ANALYZE)"?  I suspect it is a very cheap query (e.g. a single-row update), but for some reason the planner thinks it is expensive, and so JIT kicks in when it shouldn't.  Because of the high overhead, JIT is only supposed to activate for expensive queries, see jit_above_cost.

Cheers,

Jeff

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