Re: Pluggable toaster - Mailing list pgsql-hackers

On 1/14/22 19:41, Teodor Sigaev wrote:
> 
>> In my understanding, we want to be able to
>> 1. Access data from a toasted object one slice at a time, by using
>> knowledge of the structure
>> 2. If toasted data is updated, then update a minimum number of
>> slices(s), without rewriting the existing slices
>> 3. If toasted data is expanded, then allownew slices to be appended to
>> the object without rewriting the existing slices
> 
> There are more options:
> 1 share common parts between not only versions of row but between all 
> rows in a column. Seems strange but examples:
>    - urls often have a common prefix and so storing in a prefix tree (as
>      SP-GiST does) allows significantly decrease storage size
>    - the same for json - it's often use case with common part of its
>      hierarchical structure
>    - one more usecase for json. If json use only a few schemes
>      (structure) it's possible to store in toast storage only values and
>      don't store keys and structure

This sounds interesting, but very much like column compression, which 
was proposed some time ago. If we haven't made much progrees with that 
patch (AFAICS), what's the likelihood we'll succeed here, when it's 
combined with yet more complexity?

Maybe doing that kind of compression in TOAST is somehow simpler, but I 
don't see it.

> 2 Current toast storage stores chunks in heap accesses method and to 
> provide fast access by toast id it makes an index. Ideas:
>    - store chunks directly in btree tree, pgsql's btree already has an
>      INCLUDE columns, so, chunks and visibility data will be stored only
>      in leaf pages. Obviously it reduces number of disk's access for
>      "untoasting".
>    - use another access method for chunk storage
> 

Maybe, but that probably requires more thought - e.g. btree requires the 
values to be less than 1/3 page, so I wonder how would that play with 
toasting of values.

>> ISTM that we would want the toast algorithm to be associated with the
>> datatype, not the column?
>> Can you explain your thinking?
> Hm. I'll try to explain my motivation.
> 1) Datatype could have more than one suitable toasters. For different
>     usecases: fast retrieving, compact storage, fast update etc. As I
>     told   above, for jsonb there are several optimal strategies for
>     toasting:   for values with a few different structures, for close to
>     hierarchical structures,  for values with different parts by access
>     mode (easy to imagine json with some keys used for search and some
>     keys only for   output to user)
> 2) Toaster could be designed to work with different data type. Suggested
>     appendable toaster is designed to work with bytea but could work with
>     text
> 
> Looking on this point I have doubts where to store connection between 
> toaster and datatype. If we add toasteroid to pg_type how to deal with 
> several toaster for one datatype? (And we could want to has different 
> toaster on one table!) If we add typoid to pg_toaster then how it will 
> work with several datatypes? An idea to add a new many-to-many 
> connection table seems workable but here there are another questions, 
> such as will any toaster work with any table access method?
> 
> To resolve this bundle of question we propose validate() method of 
> toaster, which should be called during DDL operation, i.e. toaster is 
> assigned to column or column's datatype is changed.
> 

Seems you'd need a mapping table, to allow M:N mapping between types and 
toasters, linking it to all "compatible" types. It's not clear to me how 
would this work with custom data types, domains etc.

Also, what happens to existing values when you change the toaster? What 
if the toasters don't use the same access method to store the chunks 
(heap vs. btree)? And so on.

> More thought:
> Now postgres has two options for column: storage and compression and now 
> we add toaster. For me it seems too redundantly. Seems, storage should 
> be binary value: inplace (plain as now) and toastable. All other 
> variation such as toast limit, compression enabling, compression kind 
> should be an per-column option for toaster (that's why we suggest valid 
> toaster oid for any column with varlena/toastable datatype). It looks 
> like a good abstraction but we will have a problem with backward 
> compatibility and I'm afraid I can't implement it very fast.
> 

So you suggest we move all of this to toaster? I'd say -1 to that, 
because it makes it much harder to e.g. add custom compression method, etc.


regards

-- 
Tomas Vondra
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