On 08/13/2014 09:01 PM, Tom Lane wrote:
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.)


What does changing to lengths do to the speed of other operations?



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