Question

I would like to create a sql query (or plpgsql) that will md5() all given rows regardless of type. However, below, if one is null then the hash is null:

UPDATE thetable 
    SET hash = md5(accountid || accounttype || createdby || editedby);

I am later using the hash to compare uniqueness so null hash does not work for this use case.

The problem was the way it handles concatenating nulls. For example:

thedatabase=# SELECT accountid || accounttype || createdby  || editedby 
                 FROM thetable LIMIT 5;  

1Type113225  
<NULL>
2Type11751222 
3Type10651010 
4Type10651

I could use coalesce or CASE statements if I knew the type; however, I have many tables and I will not know the type ahead of time of every column.

Answer 1

There is much more elegant solution for this.

In Postgres, using table name in SELECT is permitted and it has type ROW. If you cast this to type TEXT, it gives all columns concatenated together in string that is actually JSON.

Having this, you can get md5 of all columns as follows:

SELECT md5(mytable::TEXT)
FROM mytable

If you want to only use some columns, use ROW constructor and cast it to TEXT:

SELECT md5(ROW(col1, col2, col3)::TEXT)
FROM mytable

Another nice property about this solution is that md5 will be different for NULL vs. empty string.

Obligatory SQLFiddle.