Unique index on hash of jsonb value - correct solution? - Mailing list pgsql-general

Hi all,

in a database I have a table with a text, a jsonb and a bytea column, which together shall be unique, like:

<snip>
  Column  |  Type  | Collation | Nullable |               Default
---------+--------+-----------+----------+-------------------------------------
  id      | bigint |           | not null | nextval('demotab_id_seq'::regclass)
  textval | text   |           | not null |
  jsonval | jsonb  |           | not null |
  blobval | bytea  |           | not null |
Indexes:
     "demo_idx" UNIQUE, btree (textval, jsonval, blobval)
</snip>

This seems to work just fine in most cases, but adding a bigger json value (the text and the bytea columns are always
short)results in a “index row size 2840 exceeds maximum 2712 for index "…"” error.  Following the hint in the error
message,I replaced the index by 

<snip>
Indexes:
     "demo_idx" UNIQUE, btree (textval, md5(jsonval::text), blobval)
</snip>

which seems to solve the issue.

My question: is this approach (a) correct and (b) still safe if the items in the jsonb (always a dict in my case) are
re-ordered? I tested a few cases, and trying to insert something like e.g. '{"a":1,"b":2}' and '{"b":2,"a":1}' actually
doesproduce the same hash (i.e. the 2nd insert attempt is rejected due to the index), but is this guaranteed by design
forevery case?  Or is there a better solution for this use case? 

Thanks in advance,
Albrecht.