Postgresql – pg_repack not organizing table by column

postgresql

I'm using pg_repack to order one table by a timestamp column, however I'm finding that after running pg_repack command it's not ordered at all.

I'm using this command

pg_repack --jobs 4 -t table_with_dates -e -o "start_time asc"

After about 10 minutes it finishes but when I query the first page of the table there is no order over "start_time" column.

I'm using postgresql 9.6.3 and pg_repack 1.4.0

Is there something i'm missing here so I can get the correct data ordering?

Edit 1:
Table definition

CREATE TABLE customer_service (
    customer_service_id character varying(32) NOT NULL,
    store_id character varying(32) NOT NULL,
    start_time timestamp without time zone NOT NULL,
    end_time timestamp without time zone NOT NULL,
    customer_id character varying(32) NOT NULL,
    service_id character varying(32) NOT NULL,
    CONSTRAINT customer_service_key PRIMARY KEY (customer_service_id),
);
CREATE INDEX customer_service_start_idx
  ON public.customer_service
  USING btree
  (start_time);

 CREATE INDEX customer_service_customer_idx
  ON public.customer_service
  USING btree(customer_id);

CREATE INDEX customer_service_service_idx
  ON public.customer_service
  USING btree
  (service_id);

SELECT performed by pg_repack:

LOG: (query) CREATE TABLE repack.table_6527203 WITH (oids = false) TABLESPACE pg_default AS SELECT customer_service_id, store_id, start_time, end_time, customer_id, service_id
FROM ONLY customer_service
ORDER BY start_time WITH NO DATA

LOG: (query) INSERT INTO repack.table_6527203 SELECT customer_service_id, store_id, start_time, end_time, customer_id, service_id
FROM ONLY customer_service

Seems like the insert into is not ordering the data?, as the order by is only present on the create table command

Edit 2:
If I run

WITH dates_per_page AS (
  SELECT (ctid::text::point)[0]::bigint AS page_number, COUNT(DISTINCT date(start_time))
  FROM customer_service
  GROUP BY 1
) SELECT avg(count) FROM dates_per_page;

I get 15.340 as the result, I would expect a number closer to 1 here…

Best Answer

This problem is fixed on the version 1.4.1 of pg_repack.

github issue for reference

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Postgresql – Postgres multiple joins slow query, how to store default child record

You write:

Each customer can have multiple sites, but only one should be displayed in this list.

Yet, your query retrieves all rows. That would be a point to optimize. But you also do not define which site is to be picked.

Either way, it does not matter much here. Your EXPLAIN shows only 5026 rows for the site scan (5018 for the customer scan). So hardly any customer actually has more than one site. Did you ANALYZE your tables before running EXPLAIN?

From the numbers I see in your EXPLAIN, indexes will give you nothing for this query. Sequential table scans will be the fastest possible way. Half a second is rather slow for 5000 rows, though. Maybe your database needs some general performance tuning?

Maybe the query itself is faster, but "half a second" includes network transfer? EXPLAIN ANALYZE would tell us more.

If this query is your bottleneck, I would suggest you implement a materialized view.

After you provided more information I find that my diagnosis pretty much holds.

The query itself needs 27 ms. Not much of a problem there. "Half a second" was the kind of misunderstanding I had suspected. The slow part is the network transfer (plus ssh encoding / decoding, possibly rendering). You should only retrieve 100 rows, that would solve most of it, even if it means to execute the whole query every time.

If you go the route with a materialized view like I proposed you could add a serial number without gaps to the table plus index on it - by adding a column row_number() OVER (<your sort citeria here>) AS mv_id.

Then you can query:

SELECT *
FROM   materialized_view
WHERE  mv_id >= 2700
AND    mv_id <  2800;

This will perform very fast. LIMIT / OFFSET cannot compete, that needs to compute the whole table before it can sort and pick 100 rows.

pgAdmin timing

When you execute a query from the query tool, the message pane shows something like:

Total query runtime: 62 ms.

And the status line shows the same time. I quote pgAdmin help about that:

The status line will show how long the last query took to complete. If a dataset was returned, not only the elapsed time for server execution is displayed, but also the time to retrieve the data from the server to the Data Output page.

If you want to see the time on the server you need to use SQL EXPLAIN ANALYZE or the built in Shift + F7keyboard shortcut or Query -> Explain analyze. Then, at the bottom of the explain output you get something like this:

Total runtime: 0.269 ms

Postgresql – Improve performance on concurrent UPDATEs for a timestamp column in Postgres

Would a different kind of column be faster? For example an integer

No. timestamp and timestamptz are just unsigned 64-bit integers internally anyway.

Is there some way to not lock the column?

It doesn't lock the column. It takes weak table lock that doesn't really block anything except DDL, and takes a row level lock on the row you're updating.

There is no way to prevent the row level lock. It exists because without it behaviour and ordering concurrent updates would be undefined. We don't like undefined behaviour in RDBMSs.

It only blocks concurrent updates of the same row anyway.

Any other tips to improve this?

Not with the detail provided. There's likely a better way to do what you're trying to do, but it'll probably involve taking a few steps back and looking for a different strategy for solving the underlying problem.

In the specific case of cache invalidation I think you might want to look into LISTEN and NOTIFY. Again though, there just isn't enough info here to go on.

Best Answer

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Postgresql – Postgres multiple joins slow query, how to store default child record

pgAdmin timing

Postgresql – Improve performance on concurrent UPDATEs for a timestamp column in Postgres

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