RE: How to solve my slow disk i/o throughput during index scan

["FREYBURGER Simon (SNCF VOYAGEURS / DIRECTION GENERALE TGV / DM RMP YIELD MANAGEMENT)" <simon.freyburger@xxxxxxx>]

Hello, and thank you again for your example !

Sorry for my late answer, I was working on a patch for our requests. I am though not completely understanding what is happening. Here is a plan of a query where I splitted the calls with OR as you suggested, what seemed to have enabled parallel scans.

https://explain.dalibo.com/plan/gfa1cf9fffd01bcg#plan/node/1

But, I still wonder, why was my request that slow ? My current understanding of what happened is :

 

• When postgresql does an Index Scan, it goes through a loop (which is not parallel) of asking for a chunk of data, and then processing it. It wait for having processed the data to ask the next chunk, instead of loading the whole index in RAM (which, I suppose, would be much faster, but also not feasible if the index is too big and the RAM too small, so postgresql does not do it). Thus, the 2MB/s.
• When it does a Bitmap Index Scan, it can parallelize disk interactions, and does not use the processor to discard lines, thus a much faster index load and processing.

 

I might be completely wrong, and would really like to understand the details, in order to explain them to my team, and to other who might encounter the same problem.  

Again, thank you very much for your help, we were really struggling with those slow queries !

Simon FREYBURGER

 

Interne

De : Andrei Lepikhov <lepihov@xxxxxxxxx>
Envoyé : vendredi 5 juillet 2024 04:05
À : FREYBURGER Simon (SNCF VOYAGEURS / DIRECTION GENERALE TGV / DM RMP YIELD MANAGEMENT) <simon.freyburger@xxxxxxx>; pgsql-performance@xxxxxxxxxxxxxxxxxxxx; Peter Geoghegan <pg@xxxxxxx>
Objet : Re: How to solve my slow disk i/o throughput during index scan

 

On 7/4/24 22: 23, FREYBURGER Simon (SNCF VOYAGEURS / DIRECTION GENERALE TGV / DM RMP YIELD MANAGEMENT) wrote: > Hello, > > Thank you, splitting in “OR” query definitely enables bitmap heap scans, > and thus parallelized read to disk

On 7/4/24 22:23, FREYBURGER Simon (SNCF VOYAGEURS / DIRECTION GENERALE 

TGV / DM RMP YIELD MANAGEMENT) wrote:

> Hello,

> 

> Thank you, splitting in “OR” query definitely enables bitmap heap scans, 

> and thus parallelized read to disk 😃! I though did not understand your 

> second point, what is parallel append, and how to enable it ?

Just for example:

DROP TABLE IF EXISTS t CASCADE;

CREATE TABLE t (id int not null, payload text) PARTITION BY RANGE (id);

CREATE TABLE p1 PARTITION OF t FOR VALUES FROM (0) TO (1000);

CREATE TABLE p2 PARTITION OF t FOR VALUES FROM (1000) TO (2000);

CREATE TABLE p3 PARTITION OF t FOR VALUES FROM (2000) TO (3000);

CREATE TABLE p4 PARTITION OF t FOR VALUES FROM (3000) TO (4000);

INSERT INTO t SELECT x % 4000, repeat('a',128) || x FROM 

generate_series(1,1E5) AS x;

ANALYZE t;

SET enable_parallel_append = on;

SET parallel_setup_cost = 0.00001;

SET parallel_tuple_cost = 0.00001;

SET max_parallel_workers_per_gather = 8;

SET min_parallel_table_scan_size = 0;

SET min_parallel_index_scan_size = 0;

EXPLAIN (COSTS OFF)

SELECT t.id, t.payload FROM t WHERE t.id % 2 = 0

GROUP BY t.id, t.payload;

  Group

    Group Key: t.id, t.payload

    ->  Gather Merge

          Workers Planned: 6

          ->  Sort

                Sort Key: t.id, t.payload

                ->  Parallel Append

                      ->  Parallel Seq Scan on p1 t_1

                            Filter: ((id % 2) = 0)

                      ->  Parallel Seq Scan on p2 t_2

                            Filter: ((id % 2) = 0)

                      ->  Parallel Seq Scan on p3 t_3

                            Filter: ((id % 2) = 0)

                      ->  Parallel Seq Scan on p4 t_4

                            Filter: ((id % 2) = 0)

Here the table is scanned in parallel. It also works with IndexScan.

-- 

regards, Andrei Lepikhov

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