Re: [GENERAL] Approach to Data Summary and Analysis

On Mon, Apr 14, 2014 at 12:27 PM, Robert DiFalco <robert.difalco@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 (

  id        BIGINT 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!