Thread: Tracking mutations in table data

Hi all,

Relatively new to PostgreSQL, looking for guidance.

I have a schema that looks like this, it's being used to log IoT events:

create table devices (
    id serial primary key,
    meta1 int,
    meta2 text
)

create table events (
    datetime timestamp,
    device_id int references devices (id),
    code int,
    primary key (datetime, device_id)
)

In addition to the IoT events themselves, I want to log the mutations
in the metadata of each device. The metadata of each device changes
much less frequently than the rate at which events are inserted,
that's why I've opted to place the data into the devices table.

My question is, what are the best practices regarding tracking this
metadata? I've done a little research into history tables and
(bi-)temporal tables, and I'm little lost based on all of the options
available. In short, I want some kind of history table of mutations in
the devices table so that I can see when the metadata is
inserted/updated/deleted.

I'm also hosting this server on a managed Azure instance, which is
somewhat limited in the number and kinds of extensions available.

Thanks in advance,
Chris

Hi Chris.

On 05/04/2020 23:06, Chris Coutinho wrote:

> In addition to the IoT events themselves, I want to log the mutations
> in the metadata of each device. The metadata of each device changes
> much less frequently than the rate at which events are inserted,
> that's why I've opted to place the data into the devices table.

I've done something similar a few times. If the application that is 
creating the device records can generate a UUID for each one I'd do all 
of that in the devices table.

create table devices (
     id serial primary key,
     meta1 int,
     meta2 text,
     identifier UUID NOT NULL,
     deleted BOOLEAN DEFAULT false,
     update_time TIMESTAMP DEFAULT now()
)

Give each device a UUID when it is added, then instead of updating or 
deleting records just create a new row with the same UUID that reflects 
the changes.

Current record for the device is:

     SELECT * FROM device
     WHERE identifier = [whatever]
     ORDER BY update_time DESC LIMIT 1;

Device history is just

     SELECT * FROM device
     WHERE identifier = [whatever]
     ORDER BY update_time;"

Deletion is just flipping a boolean, and you retain the complete history 
for the device no matter what, just because all the records are still there.

You can use triggers to block UPDATE or DELETE queries, they are core 
and won't need any additional stuff on your server. Or you can just 
trust your application :)

I've no idea how this might work at ridiculous scale, the biggest table 
I've built using this approach tops out about a million records, so bear 
that in mind.

Hope this helps,

Mark