Re: badly scaling performance with appending to bytea

Thank you Pavel for those ideas.

I should probably have mentioned we don't have access to the file
system on the PostgreSQL server, as it's provided by Amazon AWS RDS
service.

These functions look good when you can push the file to be loaded into
the database file system.

I'll see if it's possible to do this on AWS PostgreSQL RDS service but
this sort of thing is usually not

On 21 March 2018 at 12:59, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>
>
> 2018-03-21 13:56 GMT+01:00 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>:
>>
>>
>>
>> 2018-03-21 13:03 GMT+01:00 Gary Cowell <gary(dot)cowell(at)gmail(dot)com>:
>>>
>>> We are trying to implement postgresql code to load a large object into
>>> a postgresql bytea in chunks to avoid loading the file into memory in
>>> the client.
>>>
>>> First attempt was to do
>>>
>>> update build_attachment set chunk = chunk || newdata ;
>>>
>>> this did not scale and got significantly slower after 4000-5000 updates.
>>>
>>> The chunks are 4K in size, and I'm testing with a 128MB input file,
>>> requiring 32,774 chunk updates.
>>>
>>> Next, I tried creating an aggregate, thus:
>>>
>>> (taken from stackoverflow)
>>>
>>> CREATE AGGREGATE bytea_agg(bytea) (SFUNC=byteacat,STYPE=bytea);
>>>
>>> changed the code to insert the chunks to a temporary table :
>>>
>>> create temporary table build_attachment (seq bigserial primary key,
>>> chunk bytea ) on commit drop;
>>>
>>> we then insert our 4K chunks to this, which takes very little time (20
>>> seconds for the 32,774 inserts)
>>>
>>> Here's an example though of trying to select the aggregate:
>>>
>>> gary=> \timing
>>> Timing is on.
>>> gary=> select bytea_agg(chunk order by seq) from build_attachment
>>> where seq < 4000 \g output
>>> Time: 13372.843 ms
>>> gary=> select bytea_agg(chunk order by seq) from build_attachment
>>> where seq < 8000 \g output
>>> Time: 54447.541 ms
>>> gary=> select bytea_agg(chunk order by seq) from build_attachment
>>> where seq < 16000 \g output
>>> Time: 582219.773 ms
>>>
>>> So those partial aggregates completed in somewhat acceptable times but
>>> ...
>>>
>>> gary=> select bytea_agg(chunk order by seq) from build_attachment
>>> where seq < 32000 \g output
>>> this one hadn't completed in an hour - the PostgreSQL connection
>>> process for my connection on the server goes to 100% CPU and stays
>>> there, not using much RAM, not doing much IO, oddly
>>>
>>> EXPLAINing these aggregate selects doesn't show anything useful.
>>>
>>> Am I doomed to not be able to update a bytea this way? Is there some
>>> way I can tune this?
>>>
>>
>> bytea is immutable object without preallocation - so update of big tasks
>> is very expensive.
>>
>> I am thinking so using LO API and then transformation to bytea will be
>> much more effective
>>
>> \lo_import path
>>
>> you can use
>>
>> CREATE OR REPLACE FUNCTION attachment_to_bytea(attachment oid)
>> RETURNS bytea AS $$
>> DECLARE
>> fd integer;
>> size integer;
>> BEGIN
>> fd := lo_open(attachment, 262144);
>> size := lo_lseek(fd, 0, 2);
>> PERFORM lo_lseek(fd, 0, 0);
>> RETURN loread(fd, size);
>> EXCEPTION WHEN undefined_object THEN
>> PERFORM lo_close(fd);
>> RETURN NULL;
>> END;
>> $$ LANGUAGE plpgsql STRICT SECURITY DEFINER SET search_path =
>> 'pg_catalog';
>>
>> function
>>
>> import cca 44MB was in few seconds
>
>
> there is native function lo_get
>
> https://www.postgresql.org/docs/current/static/lo-funcs.html
>
>
>>
>> Regards
>>
>> Pavel
>>
>