Drop redundant index

[krzyzak]

Hi 👋

I have rather small, hobbystic project that uses your gem. I've been checking which tables are the biggest among my project, and noticed this:

SELECT
-#    relname as "Table",
-#    pg_size_pretty(pg_total_relation_size(relid)) As "Size",
-#    pg_size_pretty(pg_total_relation_size(relid) - pg_relation_size(relid)) as "External Size"
-#    FROM pg_catalog.pg_statio_user_tables ORDER BY pg_total_relation_size(relid) DESC;
┌───────────────────────────────┬─────────┬───────────────┐
│             Table             │  Size   │ External Size │
├───────────────────────────────┼─────────┼───────────────┤
│ que_scheduler_audit           │ 72 MB   │ 36 MB         │

For my amount of rows (given how simple single row is)

select count(*) from que_scheduler_audit;
┌────────┐
│ count  │
├────────┤
│ 846371 │
└────────┘
(1 row)

Time: 67.357 ms

I started investigating what exactly takes that much space.

 \d que_scheduler_audit;
                       Table "public.que_scheduler_audit"
┌──────────────────┬──────────────────────────┬───────────┬──────────┬─────────┐
│      Column      │           Type           │ Collation │ Nullable │ Default │
├──────────────────┼──────────────────────────┼───────────┼──────────┼─────────┤
│ scheduler_job_id │ bigint                   │           │ not null │         │
│ executed_at      │ timestamp with time zone │           │ not null │         │
└──────────────────┴──────────────────────────┴───────────┴──────────┴─────────┘
Indexes:
    "que_scheduler_audit_pkey" PRIMARY KEY, btree (scheduler_job_id)
    "index_que_scheduler_audit_on_scheduler_job_id" UNIQUE, btree (scheduler_job_id)
Referenced by:
    TABLE "que_scheduler_audit_enqueued" CONSTRAINT "que_scheduler_audit_enqueued_scheduler_job_id_fkey" FOREIGN KEY (scheduler_job_id) REFERENCES que_scheduler_audit(scheduler_job_id)

It seems that que_scheduler_audit have both PRIMARY KEY constraint index AND UNIQUE index on the same column, meaning that the latter one is essentially redundant, thus I created a PR that drops it.
After that change, I saved 18MB in my simple application:

┌───────────────────────────────┬─────────┬───────────────┐
│             Table             │  Size   │ External Size │
├───────────────────────────────┼─────────┼───────────────┤
│ que_scheduler_audit           │ 54 MB   │ 18 MB         │

[hlascelles]

Cheers very much for this @krzyzak. A perfect report... Really, it looks like you spent more time on the issue than on the fix! 👍

You are absolutely right, it looks redundant. I will have a look at getting this in as soon as possible. Thank you for you contribution!

[hlascelles]

Tests added to ensure one isn't readded: #191. Specifically here.

[hlascelles]

If you're interested in saving more space you can now run a built in audit clear down job: #193