Re: Zedstore - compressed in-core columnar storage

On 10/04/2019 10:38, Konstantin Knizhnik wrote:
> I also a little bit confused about UNDO records and MVCC support in
> Zedstore. Actually columnar store is mostly needed for analytic for
> read-only or append-only data. One of the disadvantages of Postgres is
> quite larger per-record space overhead caused by MVCC.
> It may be critical if you want to store huge timeseries  with relatively
> small number of columns (like measurements of some sensor).
> It will be nice to have storage format which reduce this overhead when
> it is not needed (data is not updated).

Sure. Definitely something we need to optimize.

> Right now, even without UNFO pages, size of zedstore is larger than size
> of original Postgres table.
> It seems to be very strange.

If you have a table with a lot of columns, but each column is small,
e.g. lots of boolean columns, the item headers that zedstore currently
stores for each datum take up a lot of space. We'll need to squeeze
those harder to make this competitive. Instead of storing a header for
each datum, if a group of consecutive tuples have the same visibility
information, we could store the header just once, with an array of the
datums, for example.

- Heikki