Josh Berkus: Stupid Hacks: Dictionary replace function

I write a lot more PL/pgSQL than I'd like to.  Not that I don't like SQL, but as a language PL/pgSQL really shows its thrown-together origin; it's like 80's primitive.  One thing that PL/pgSQL lacks is good string manipulation tools, which is particularly tragic given that the #1 thing to do in PL/pgSQL is to generate queries from parameters and run EXECUTE.

Postgres has two built-in ways to do string substitution: concatenation and format().  Both have drawbacks.  Let me give you an example:

EXECUTE 'SELECT ' || col1 || ', ' || col2 || ' FROM ' || userschema ||
  '.accounts WHERE ' || filterclause || ' ORDER BY ' || col1 || ',' || col2;

EXECUTE format('SELECT %s, %s FROM %s.accounts WHERE %s ORDER BY %s, %s', col1, col2, userschema, filterclause, col1, col2);

You can see the problem here.  Both formats are hard to read and hard to maintain.  Python and Perl have a good fix for this: dictionary/hash-based string substitution, where you can swap in the dictionary keys for the values.  So I wrote up a quick hack to do this in PL/pgSQL.

Here's the dict-replace function.

Using it, you'd replace the above with:

EXECUTE replace_vars('SELECT ${col1}, ${col2} FROM ${userschema}.accounts
  WHERE ${filterclause} ORDER BY ${col1}, ${col2}', vardict);

Of course, you need to first set up the vardict as a JSON value, in the form:

vardict := '{ "col1" : "username", 'col2' : "branchname", ...}'

Still, much more readable, eh?  No refcounting, no repeating variables, no string breaks.  I used Bash's variable substitution syntax of ${var} because it seemed like the thing least likely to conflict with user text, unlike anything involving %.

There's some caveats, though: it's not performant, and you could probably find strings which will break it, particularly if you're swapping in JSON values.  You still have to do your own quoting of strings.  And if you have access to PL/Perl or PL/Python you don't need this nonsense.

But for the minimal case, it should help.