Re: jsonb format is pessimal for toast compression

I wrote:
> That's a fair question.  I did a very very simple hack to replace the item
> offsets with item lengths -- turns out that that mostly requires removing
> some code that changes lengths to offsets ;-).  I then loaded up Larry's
> example of a noncompressible JSON value, and compared pg_column_size()
> which is just about the right thing here since it reports datum size after
> compression.  Remembering that the textual representation is 12353 bytes:

> json:                382 bytes
> jsonb, using offsets:        12593 bytes
> jsonb, using lengths:        406 bytes

Oh, one more result: if I leave the representation alone, but change
the compression parameters to set first_success_by to INT_MAX, this
value takes up 1397 bytes.  So that's better, but still more than a
3X penalty compared to using lengths.  (Admittedly, this test value
probably is an outlier compared to normal practice, since it's a hundred
or so repetitions of the same two strings.)
        regards, tom lane