问题
I got the answer to check for one certain BOM in a PostgreSQL text column. What I really like to do would be to have something more general, i.e. something like
select decode(replace(textColumn, '\\', '\\\\'), 'escape') from tableXY;
The result of a UTF8 BOM is:
\357\273\277
Which is octal bytea and can be converted by switching the output of bytea in pgadmin:
update pg_settings set setting = 'hex' WHERE name = 'bytea_output';
select '\357\273\277'::bytea
The result is:
\xefbbbf
What I would like to have is this result as one query, e.g.
update pg_settings set setting = 'hex' WHERE name = 'bytea_output';
select decode(replace(textColumn, '\\', '\\\\'), 'escape') from tableXY;
But that doesn't work. The result is empty, probably because the decode cannot handle hex output.
回答1:
If the final purpose is to get the hexadecimal representation of all the bytes that constitute the strings in textColumn, this can be done with:
SELECT encode(convert_to(textColumn, 'UTF-8'), 'hex') from tableXY;
It does not depend on bytea_output. BTW, this setting plays a role only at the final stage of a query, when a result column is of type bytea and has to be returned in text format to the client (which is the most common case, and what pgAdmin does). It's a matter of representation, the actual values represented (the series of bytes) are identical.
In the query above, the result is of type text, so this is irrelevant anyway.
I think that your query with decode(..., 'escape') can't work because the argument is supposed to be encoded in escape format and it's not, per comments it's normal xml strings.
回答2:
With the great help of Daniel-Vérité I use this general query now to check for all kind of BOM or unicode char problems:
select encode(textColumn::bytea, 'hex'), * from tableXY;
I had problem with pgAdmin and too long columns, as they had no result. I used that query for pgAdmin:
select encode(substr(textColumn,1,100)::bytea, 'hex'), * from tableXY;
Thanks Daniel!
来源：https://stackoverflow.com/questions/23342291/how-to-convert-postgresql-escape-bytea-to-hex-bytea