sql/index/pilosa: return all index ids in lookups

[erizocosmico]

Fixes #585

[kuba--]

Maybe it should be map[string]int, where key is an index name and value will be order id.
So we'll have auto-uniques (because of map), and then we can sort map entries by value, sth. like:

m := make(map[string]int)
m[indexName] = len(m)
//...
indexes := make([]string, len(m))
for k, v := range m {
    index[v] = k
}
//...
return indexes

WDYT?

[erizocosmico]

Why the int? We can just use a map[string]struct{} and then sort them.

Also, even if indexes is a map, it may not be unique, because id can be inside that map.

[kuba--]

int just because we can store in what order we "got" indexes, so at the end you can just re-write it into the slice in the values order (instead of sorting, first).

[kuba--]

The question is how do we want to show indexes (sorted in the order how filter was parsed, or sorted by name)?
On the other hand, these are "small" numbers so most likely keep the most readable version.

[erizocosmico]

I will use the struct{} only because the int requires some explanation at first sight and the other doesn't. Regarding the order, I think it does not matter much, so we can just sort.

The thing is: with a map we start requiring constructors and so on and before you could just &indexLookup{id: whatever}

[kuba--]

Ok, so let's keep it what we have so far. I suppose, there is no many cases where we'll use the same index more than once, so maybe map can be kind of overhead.

[ajnavarro]

LGTM after @kuba-- comments

[erizocosmico]

Done, though I think it unnecessarily complicates the code for very little gain.

[kuba--]

Btw, we have to fix old "merge" tests: https://github.com/src-d/go-mysql-server/pull/586/files#diff-136ffc4118af2fd83a7cbf2bc27fe357R215