When merging streams of system iterators we don't use tags or time.
Instead we add series keys (in the case of, for example, `SHOW SERIES`)
to the `Aux` field of the iterators' elements. This is because we only
emit merged and sorted sets of series key to the client.
We currently use `SortedMergeHeap`s to merge together multiple
iterators, and the comparitor function did not consider `Aux` fields
when determining which heap to pop the next item off during a merge. As
such, `SHOW SERIES` and `SHOW TAG KEYS` (any meta query that gets
converted into a special type of `SELECT`) were returning results in
arbitrary order.
This issue was never noticed on the `inmem` index because the streams
are always duplicates of each other, and of course it doesn't matter if
you arbitrarily merge together two idential, sorted streams...
The issue first manifested itself on the `tsi1` index, but this fix will
apply to both indexes.
This change provides a clear separation between the query engine
mechanics and the query language so that the language can be parsed and
dealt with separate from the query engine itself.
Edd Robinson
Jonathan A. Sternberg