PostgreSQL – Delete from Table Causes Database to Enter Recovery Mode

postgresql

It's becoming common to see this problem in the development environment.
My application is not in production yet, but I'm worried this problem could happen.

As I develop new features, I commonly clear some tables when necessary.
I started to get into this problem with one specific table I try to clean.
I delete everything from other tables that could have foreign constraints, but when I try to delete from that specific table, pgAdmin loses the connection and the database enters in recovery mode.

First I get a warming saying:

"The application has lost the database connection".

If I try several times to connect and execute again, then I get a message

"FATAL: the database system is in recovery mode".

One thing that worked was to execute an update setting some field to it's own value on all rows (i.e update table set field = field).
After this I could delete all rows.

I'm actually using c# and EntityFramework 6 with the DevArt.PostgreSql plugin to control the database creation, but I generated the table script from pgAdmin to help understand the table structure. I also added one other table that has a foreign key to the table in question.

The application domain, code and database structure is written in pt-BR, so I tried to translate the table and fields names to help.
It's an application to control order requests and charges.

Table (request_event):

CREATE TABLE public.request_event
(
    id integer NOT NULL DEFAULT nextval('request_event_id_seq'::regclass),
    id_request integer NOT NULL,
    type integer NOT NULL,
    date timestamp without time zone NOT NULL,
    time_informed boolean NOT NULL,
    id_user integer,
    message text COLLATE pg_catalog."default" NOT NULL,
    visible boolean NOT NULL,
    occurrence timestamp without time zone NOT NULL DEFAULT '0001-01-01 00:00:00'::timestamp without time zone,
    CONSTRAINT request_event_pkey PRIMARY KEY (id),
    CONSTRAINT "FK_request_event_request_id_request" FOREIGN KEY (id_request) REFERENCES public.request (id) MATCH SIMPLE ON UPDATE NO ACTION ON DELETE NO ACTION,
    CONSTRAINT "FK_request_event_user_id_user" FOREIGN KEY (id_user) REFERENCES public.user (id_person) MATCH SIMPLE ON UPDATE NO ACTION ON DELETE NO ACTION
)
WITH (
    OIDS = FALSE
)
TABLESPACE pg_default;

ALTER TABLE public.request_event OWNER to postgres;

CREATE INDEX "IX_request_event_id" ON public.request_event USING btree (id) TABLESPACE pg_default;
CREATE INDEX "IX_request_event_id_request" ON public.request_event USING btree (id_request) TABLESPACE pg_default;
CREATE INDEX "IX_request_event_id_user" ON public.request_event USING btree (id_user) TABLESPACE pg_default;

Table (charge_event):

CREATE TABLE public.charge_event
(
    id integer NOT NULL,
    id_charge integer NOT NULL,
    CONSTRAINT charge_event_pkey PRIMARY KEY (id),
    CONSTRAINT "FK_charge_event_charge_id_charge" FOREIGN KEY (id_charge) REFERENCES public.charge (id) MATCH SIMPLE ON UPDATE NO ACTION ON DELETE NO ACTION,
    CONSTRAINT "FK_charge_event_request_event_id" FOREIGN KEY (id) REFERENCES public.request_event (id) MATCH SIMPLE ON UPDATE NO ACTION ON DELETE NO ACTION
)
WITH (
    OIDS = FALSE
)
TABLESPACE pg_default;

ALTER TABLE public.charge_event OWNER to postgres;
CREATE INDEX "IX_charge_event_id" ON public.charge_event USING btree (id) TABLESPACE pg_default;
CREATE INDEX "IX_charge_event_id_charge" ON public.charge_event USING btree (id_charge) TABLESPACE pg_default;

Delete SQL (fail):

DELETE FROM charge_event;
DELETE FROM request_event;

Delete SQL (worked):

DELETE FROM charge_event;
UPDATE request_event SET type = type;
DELETE FROM request_event;

Best Answer

As suggested by @a_horse_with_no_name, there is one bugfix related to crashes with DELETE statements here and here.

I waited for the problem to occur again. After I updated to version 11.1, the delete statement worked as expected.

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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 – Simple PostgreSQL lookup table is inexplicably slow

Rather than "VARCHAR(40000)" why not use "TEXT"?
HASH index use is discouraged (see docs at http://www.postgresql.org/docs/current/static/indexes-types.html).
Have you run "ANALYZE" on your tables before running the query?
Giant IN lists can be performance killers.

How do the following queries perform?

SELECT e.entityId
FROM entities e
INNER JOIN triples t ON (t.object = e.entityId)
LIMIT 10000;

SELECT e.entityId
FROM entities e
WHERE EXISTS (SELECT 1 FROM triples t WHERE t.object = e.entityId LIMIT 10000);

Best Answer

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

pgAdmin timing

Postgresql – Simple PostgreSQL lookup table is inexplicably slow

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