Add index on salesrule_coupon

[luigifab]

Description

We have 220,000 coupons in salesrule_coupon.
This PR add an index on code column, like #2447.

Here is a before/after with Blackfire:

Contribution checklist

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[tmotyl]

what query is being run?

[luigifab]

Yes, I forgot to copy/paste: select salesrule_coupon.* from salesrule_coupon wehere (salesrule_coupon.code = ?)

[kiatng]

The column code is

UNIQUE KEY `UNQ_SALESRULE_COUPON_CODE` (`code`),

According to this stackoveflow, it's unnecessary to index it again. I am not sure how to interpret Blackfire output, is stackoverflow wrong?

[edit]

The unique index serves a dual purpose. It functions just like any other index when you perform a query based on a phone number: ...However, it also checks every value when attempting to insert or update a record to ensure that the value doesn’t already exist. In this way, the unique index acts as a constraint.

Unique indexes use as much space as nonunique indexes do. The value of every column as well as the record’s location is stored. This can be a waste if you use the unique index as a constraint and never as an index. Put another way, you may rely on the unique index to enforce uniqueness but never write a query that uses the unique value. In this case, there’s no need for MySQL to store the locations of every record in the index: you’ll never use them.

Unfortunately, there’s no way to signal your intentions to MySQL. In the future, we’ll likely find a feature introduced for this specific case. The MyISAM storage engine already has support for unique columns without an index (it uses a hash-based system), but the mechanism isn’t exposed at the SQL level yet.

From https://www.oreilly.com/library/view/high-performance-mysql/0596003064/ch04.html

[luigifab]

Damned, you are right. Now I remember that the store use a module that removed the unique key to allow multiple coupon with same code.

[fballiano]

that's @kiatng for the finding!

I'm closing this one but I still think we should merge #2447

[sreichel]

but I still think we should merge #2447

We already merged it.

[fballiano]

I was meaning #1110 :-D