Re: Memory Issue with array_agg? - Mailing list pgsql-general

Hi Pavel,

What kind of example do you need?  I cant give you the actual data I have in the table, but I can give you an example query and the schema attached below.  From there, I would just put in 2 million rows worth 1.2 Gigs of data.  Average size of the the extended columns (using the pg_column_size function) in bytes are:

guid: 33

name: 2.41

currency: 4

fields: 120.32

example query:

-- find duplicate records using a guid

select guid, array_agg(id) from orders group by guid;

example schema:

                                     Table "public.things"                                                        

   Column   |            Type             |                      Modifiers                      | Storage  | Stats target | Description 

------------+-----------------------------+-----------------------------------------------------+----------+--------------+-------------

 id         | integer                     | not null default nextval('things_id_seq'::regclass) | plain    |              | 

 version    | integer                     | not null                                            | plain    |              | 

 created_at | timestamp without time zone | not null                                            | plain    |              | 

 updated_at | timestamp without time zone | not null                                            | plain    |              | 

 foo_id     | integer                     | not null                                            | plain    |              | 

 bar_id     | integer                     | not null                                            | plain    |              | 

 baz_id     | integer                     | not null                                            | plain    |              | 

 guid       | character varying           | not null                                            | extended |              | 

 name       | character varying           | not null                                            | extended |              | 

 price      | numeric(12,2)               | not null                                            | main     |              | 

 currency   | character varying           | not null                                            | extended |              | 

 amount     | integer                     | not null                                            | plain    |              | 

 the_date   | date                        | not null                                            | plain    |              | 

 fields     | hstore                      |                                                     | extended |              | 

Indexes:

    "things_pkey" PRIMARY KEY, btree (id)

    "things_foo_id_idx" btree (foo_id)

    "things_bar_id_idx" btree (bar_id)

    "things_baz_id_idx" btree (baz_id)

    "things_guid_uidx" UNIQUE, btree (guid)

    "things_lpad_lower_name_eidx" btree (lpad(lower(name::text), 10, '0'::text))

    "things_price_idx" btree (price)

    

Foreign-key constraints:

    "things_foo_id_fkey" FOREIGN KEY (foo_id) REFERENCES foos(id)

    "things_bar_id_fkey" FOREIGN KEY (bar_id) REFERENCES bars(id)

    "things_baz_id_fkey" FOREIGN KEY (baz_id) REFERENCES bazs(id)

Triggers:

    timestamps_trig BEFORE INSERT OR UPDATE ON things FOR EACH ROW EXECUTE PROCEDURE timestamps_tfun()

Let me know if you need anything else.

Thanks,

On Mon, Aug 19, 2013 at 3:29 AM, Pavel Stehule <pavel.stehule@gmail.com> wrote:

Hello

please, can you send some example or test?

Regards

Pavel Stehule

2013/8/19 Robert Sosinski <rsosinski@ticketevolution.com>

When using array_agg on a large table, memory usage seems to spike up until Postgres crashes with the following error:

2013-08-17 18:41:02 UTC [2716]: [2] WARNING: terminating connection because of crash of another server process
2013-08-17 18:41:02 UTC [2716]: [3] DETAIL: The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
2013-08-17 18:41:02 UTC [2716]: [4] HINT: In a moment you should be able to reconnect to the database and repeat your command.

I've definitely isolated it down to using array_agg, as when I changed the query to use string_agg, it worked fine.  I also tried using array_agg on a few different queries, all yielding the same issue.  Swapping in string_agg fixed the issue once more.

This particular table has over 2 million rows and is 1.2 Gigs, and when I ran the query while viewing htop, the virtual size of the Postgres process ballooned to 13.9G until crashing.

The version of Postgres I am using is: PostgreSQL 9.2.4 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2, 64-bit

Any help would be much appreciated, thanks!

-Robert