Re: Schema for caching message-count in folders using triggers

På torsdag 05. mars 2015 kl. 20:59:28, skrev Adrian Klaver <
adrian(dot)klaver(at)aklaver(dot)com <mailto:adrian(dot)klaver(at)aklaver(dot)com>>: On 03/05/2015
11:45 AM, Andreas Joseph Krogh wrote:
> Hi all.
> I'm facing a problem with my current schema for email where folders
> start containing several 100K of messages and count(*) in them taks
> noticeable time. This schema is accessible from IMAP and a web-app so
> lots of queries of the type "list folders with message count" are performed.
> So, I'm toying with this idea of caching the message-count in the
> folder-table itself.
> I currently have this:
>
> CREATE or replace FUNCTIONcount_increment_tf()RETURNS TRIGGER AS$_$
> BEGIN
> UPDATE folder SETmessage_count  =message_count  + 1 WHEREid  =NEW.folder_id;
> RETURNNEW;
> END $_$LANGUAGE'plpgsql';
>
> CREATE or replace FUNCTIONcount_decrement_tf()RETURNS TRIGGER AS$_$
> BEGIN
>      UPDATE folder SETmessage_count  =message_count  - 1 WHEREid 
=OLD.folder_id;
> RETURNOLD;
> END $_$LANGUAGE'plpgsql';
>
> CREATE or replace FUNCTIONcount_update_tf()RETURNS TRIGGER AS$_$
> BEGIN
>      UPDATE folder SETmessage_count  =message_count  - 1 WHEREid 
=OLD.folder_id;
>      UPDATE folder SETmessage_count  =message_count  + 1 WHEREid 
=NEW.folder_id;
> RETURNNEW;
> END $_$LANGUAGE'plpgsql';
>
> CREATE TRIGGERincrement_folder_msg_tAFTER INSERT ON message FOR EACH ROW
EXECUTE PROCEDUREcount_increment_tf();
> CREATE TRIGGERdecrement_folder_msg_tAFTER DELETE ON message FOR EACH ROW
EXECUTE PROCEDUREcount_decrement_tf();
> CREATE TRIGGERupdate_folder_msg_tAFTER UPDATE ON message FOR EACH ROW
EXECUTE PROCEDUREcount_update_tf();
>
> The problem with this is locking (waiting for another TX to commit when
> updating the same folder) and deadlock issues when trying to
> simultaneously insert/delete/update messages  in a folder.
> Does anyone have any better ideas for safely caching the message-count
> in each folder without locking and deadlock issues?

How accurate does this have to be?

Not exactly following what is folder?
Is it a table that contains the messages?

A top of the head idea would be to use sequences. Create a sequence for
each folder starting at current count and then use nextval, setval to
change the value:

http://www.postgresql.org/docs/9.4/interactive/functions-sequence.html

It is not transactional, so it would probably not be spot on, which is
why I asked about accuracy earlier.   Yes, 'folder' is a table which contains
'message':   create table folder( id serial PRIMARY KEY, name varchar not null
unique, message_count integer not null default 0 ); create table message( id
serial PRIMARY KEY, folder_id INTEGER NOT NULL REFERENCES folder(id), message
varchar not null);   The count has to be exact, no estimate from EXPLAIN or
such...   -- Andreas Joseph Krogh CTO / Partner - Visena AS Mobile: +47 909 56
963 andreas(at)visena(dot)com <mailto:andreas(at)visena(dot)com> www.visena.com
<https://www.visena.com> <https://www.visena.com>