Columnar databases are not "already "indexed"". Their advantage instead comes from their ability to only load the relevant parts of rows when doing scans.
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.
What does it even mean to be "halfway" to the structure of an index? Do they allow filtering a subset of rows with a complexity that's less than linear in the total number of rows or not?
A row-based index is a column-wise copy of the data, with mechanisms to skip forward during scanning. You maintain a separate copy of the column to support this, making indexes expensive, and thus the DBA is asked to maintain a minimal 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.
> 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.
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.
Zonemap / skip indexes don’t require sorting, still provide significantly improved searching over full tablescans, and typically applied to every column by default. 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 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