Re: TOAST Fields serialisation/deserialization performance

On Wed, 2023-07-26 at 18:15 +0530, Piyush Katariya wrote:
> I have a few queries regarding the TOAST Fields serialisation/deserialization performance.
>
> The use case i am trying to solve here is to have millions of partitions and aggregate the data in array field.
>
> I wish to know if i declare certain column in table as "array of UDT/JSONB" and enable
> either lz4 or zstd compression on it, does appending or prepending to that array or even
> changing the intermediate fields of UDT/JSONB objects. in that array has a runtime cost
> of full array data de-serialization every single time. If i perform any UPDATE operation
> on its elements or add/remove new elements from any position, does PG rewrites the new
> version of the column value regardless of its size.

Updating even a small part of a large JSONB value requires that the entire thing is
read and written, causing a lot of data churn.

This is inefficient, and you shouldn't use large JSONB values if you plan to do that.

If the data have a regular structure, use a regular relational data model.
Otherwise, one idea might be to split the JSONB in several parts and store each
of those parts in a different table row.  That would reduce the impact.

Yours,
Laurenz Albe