Sam Mason <sam@samason.me.uk> writes:
> You just want an extra set of brackets; i.e.:
> CREATE INDEX token_position_func ON token ((position+1));
> Should do the trick. Not entirely sure why, but it'll probably have
> something to do with avoiding ambiguity in the grammar.
Right. The problem is the Berkeley-era decision to put index opclasses
into the syntax without any keyword or punctuation, viz
create index ... on table (column_name [ opclass_name ]);
So something like
CREATE INDEX token_position_func ON token (a + b);
is ambiguous: is the + an infix operator, or is it a postfix operator
and the "b" is to be taken as an opclass name?
We hacked around that by requiring parens around expressions. For
backwards compatibility with other Berkeley-era syntax, there's
a special exception that you can omit the parens when the expression is
just a function call.
regards, tom lane
