[gruez]

>Use bigint, never UUID. UUIDs are massive (2x a bigint) and now your DBMS has to copy that enormous value to every side of a relation.

"enormous value" = 128 bits (compared to 64 bits)

In the worst case this causes your m2m table to double, but I doubt this has a significant impact on the overall size of the DB.

[TylerE]

The concern isn’t the sign of the db on disc but doubling the size of all the indexes in memory

[sgarland]

Or if it’s MySQL, the PK is implicitly copied into every secondary index. Adds up quickly.

[pyuser583]

Wow did not know that. MySQL has tons of hidden behavior.

[sgarland]

So does Postgres to an extent, but in general, MySQL has more hidden edge cases, and Postgres has more hidden maintenance requirements.

[sgarland]

When you have a few million rows, no. When you have hundreds of millions or billions of rows, yes, it matters very much.

[LunaSea]

In many-to-many tables, the per-row overhead of the DBM usually weighs much more than the actual column data.