On Tue, Apr 15, 2014 at 10:56 AM, Chris Curvey ch...@chriscurvey.comwrote:
On Mon, Apr 14, 2014 at 12:27 PM, Robert DiFalco robert.difa...@gmail.com
wrote:
I have several related tables that represent a call state. Let's think of
these as phone calls to simplify things. Sometimes I need to determine the
last time a user was called, the last time a user answered a call, or the
last time a user completed a call.
The basic schema is something like this:
CREATE TABLE calls (
id BIGINT NOT NULL, // sequence generator
user_id BIGINT NOT NULL,
called TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (id),
FOREIGN KEY (user_id) REFERENCES my_users(id) ON DELETE CASCADE
);
CREATE TABLE calls_answered (
idBIGINT NOT NULL,
answered TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (id),
FOREIGN KEY (id) REFERENCES calls(id) ON DELETE CASCADE
);
And so on for calls_connected, calls_completed, call_errors, etc.
Occasionally I will want to know things like When was the last time a
user answered a call or How many times has a user been called.
I can do these queries using a combination of MAX or COUNT. But I'm
concerned about the performance.
SELECT MAX(a.id)
FROM calls_answered a JOIN calls c ON c.id = a.id
WHERE c.user_id = ?;
Or the number of answered calls:
SELECT MAX(a.id)
FROM calls_answered a JOIN calls c ON c.id = a.id
WHERE c.user_id = ?;
Sometimes I might want to get this data for a whole bunch of users. For
example, give me all users whose have not answered a call in the last 5
days. Or even what percentage of users called actually answered a call.
This approach could become a performance issue. So the other option is to
create a call_summary table that is updated with triggers.
The summary table would need fields like user_id, last_call_id,
call_count, last_answered_id, answered_count, last_completed_id,
last_completed_count, etc.
My only issue with a summary table is that I don't want a bunch of null
fields. For example, if the user was *called* but they have never
*answered* at call then the last_call_id and call_count fields on the
summary table would be non-NULL but the last_answer_id and answer_count
fields WOULD be NULL. But over time all fields would eventually become
non-NULL.
So that leads me to a summary table for EACH call state. Each summary
table would have a user id, a ref_id, and a count -- one summary table for
each state e.g. call_summary, call_answered_summary, etc.
This approach has the down side that it creates a lot of tables and
triggers. It has the upside of being pretty efficient without having to
deal with NULL values. It's also pretty easy to reason about.
So for my question -- is the choice between these a personal preference
sort of thing or is there a right or wrong approach? Am I missing another
approach that would be better? I'm okay with SQL but I'm not expert so I'm
not sure if there is an accepted DESIGN PATTERN for this that I am missing.
Thanks!
(Sorry, fat-fingered and hit send too early...)
CREATE TABLE calls (
id BIGINT NOT NULL, // sequence generator
user_id BIGINT NOT NULL,
called TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
answered TIMESTAMPTZ NULL,
connected TIMESTAMPTZ NULL,
completed TIMESTAMPTZ NULL,
PRIMARY KEY (id),
FOREIGN KEY (user_id) REFERENCES my_users(id) ON DELETE CASCADE
);
Then your queries end up looking like this:
--last time john answered
SELECT MAX(a.id)
FROM calls
where answered is not null
and user_id = ?
-- users that have not answered a call in the last five days (I can think
of a few ways to interpret that phrase)
select myusers.*
from myusers
where not exists
( select *
from calls
where calls.user_id = myusers.user_id
and answered = five days ago)
-- average ring time
select avg(extract ('seconds' from called - answered))
where answered is not null
--
I asked the Internet how to train my cat, and the Internet told me to get a
dog.
