They’re indexed in the sense that they’re already halfway to the structure of an index — which is why they’re happy to toss indexes on top arbitrarily, instead of demanding the user to manage a minimum subset.
A columnar database’s index is simply laid out on top of the column data. If the column is the key, then it’s sorted by definition, and no index is really required outside of maybe a zone map, because you can binary search. A non-key column gets a zone map / skip index laid out on top, which is cheap to maintain… because it’s already a column-wise slice of the data.
You don’t often add indexes to an OLAP system because every column is indexed by default — because it’s cheap to maintain, because you don’t need a separate column-wise copy of the data because it’s already a column-wise copy of the data.
I don't see how that's different from storing a traditional index. You can't just lay it on top of the column, because the column is stored in a different order than what the index wants.
In a row-based rdbms, any indexing whatsoever is a copy of the column-data, so you might as well store it sorted every time. It’s not inherent to the definition.
That's still a separate index though, no? It's not intrinsic in the column storage itself, although I guess it works best with it if you end up having to do a full-scan of the column section anyway.
> Sorting is even better, just at the cost of a second copy of the dataset. > ... > In a row-based rdbms, any indexing whatsoever is a copy of the column-data
So the same thing, no?
> So the same thing, no? Consider it as like: for a given filtered-query, a row-based storage is doing a table-scan if no index exists. There is no middle ground. Say 0% value or 100%.
A columnar database’s baseline is a decent index, and if there’s a sorted index then even better. Say 60% value vs 100%.
The relative importance of having a separate, explicit, sorted index is much lower in a columnar database, because the baseline is different. (Although maintaining extra sorted indexes is a columnar database is much more expensive — you basically have to keep a second copy of the entire table sorted on the new key(s))