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help / color / mirror / Atom feedFrom: Adrian Klaver <adrian.klaver@aklaver.com>
To: Andreas Joseph Krogh <andreas@visena.com>
To: pgsql-sql@postgresql.org <pgsql-sql@postgresql.org>
Subject: Re: Schema for caching message-count in folders using triggers
Date: Thu, 05 Mar 2015 11:59:28 -0800
Message-ID: <54F8B5A0.4020607@aklaver.com> (raw)
In-Reply-To: <VisenaEmail.7.3433a658591afa2a.14beb71d3ac@tc7-visena>
References: <VisenaEmail.7.3433a658591afa2a.14beb71d3ac@tc7-visena>
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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.
> Thanks.
> --
> *Andreas Joseph Krogh*
> CTO / Partner - Visena AS
> Mobile: +47 909 56 963
> andreas@visena.com <mailto:andreas@visena.com>
> www.visena.com <https://www.visena.com;
> <https://www.visena.com;
--
Adrian Klaver
adrian.klaver@aklaver.com
--
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To: pgsql-sql@postgresql.org
Cc: adrian.klaver@aklaver.com, andreas@visena.com
Subject: Re: Schema for caching message-count in folders using triggers
In-Reply-To: <54F8B5A0.4020607@aklaver.com>
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