For small fixed-size structures, it's better to use a slice of values, instead
of a slice of pointers to values: they're stored contiguously and thus can be
iterated on quickly by the CPU, and it does remove an indirection per object
every time the GC kicks in.
Apparently, postgresql isn't smart enough to realize that once a true value
value is found as part of a `SELECT true`, there is no need to scan the rest of
the table, so we have to make this explicit. We could also have used the
`SELECT EXISTS(…)` construct, but it's more verbose and I don't like it.
