I'm in need of an aggregate hash function. Something like "select md5_agg(someTextColumn) from (select someTextColumn from someTable order by someOrderingColumn)". I know that there is an existing MD5 function, but it is not an aggregate. I have thought about writing a "concat" aggregate function that would concatenate the input into a long string and then using MD5() on that, but that seems like it would have some bad performance implications (memory consumption, possibly spilling to disk, many large memory copies, etc) as it would buildup the entire concatenated string first before hashing it.
I also thought about making a aggregate function that works by keeping the MD5 result as a string in the state, then concatenating the new input with the current state, hashing that and using it as the new state. This solves the problem of building up a giant string to just traverse over at the end to get the MD5 sum. This approach would actually work for me, but it doesn't give me the actual MD5 sum of the data which is what I really want.