Re: Can we get sha* function over text, that could be used in index?

Linus Heckemann wrote:

> - there is a byte-array representation of text columns, which appears to
> be independent of database encoding

Not sure what you're refering to. Both the on-disk and in-memory
representations of text/varchar are encoding-dependent.

> The obvious (to a naive user, like I was) approach, casting to bytea,
> has exceptionally surprising behaviour: for many text strings, it does
> exactly what the naive user might hope for, giving back the UTF-8
> representation. But multiple distinct text strings, like '\033' and
> '\x1b', convert to the same byte string! And text strings containing a
> backslash that doesn't fit the bytea hex format or the bytea escape
> format will fail to convert completely!

Yes. It seems a common mistake to forget or ignore that
backslashes are special in the input text representation of bytea.
It might be not obvious from reading the doc at [1]
but we just need to quote backslashes by doubling them.

AFAIK a working solution for the OP would be:
sha256(replace(colname, '\', '\\')::bytea)

The result is encoding-dependent, but that does not matter
in the context of an expression. index.
If the database ever needs to change its encoding, it will have to
be recreated entirely anyway.

[1]
https://www.postgresql.org/docs/current/datatype-binary.html#DATATYPE-BINARY-BYTEA-ESCAPE-FORMAT

Best regards,
--
Daniel Vérité
https://postgresql.verite.pro/