use a different prefix than `@` to represent variables

**Describe the bug**

in postgresql `@` is an operator for abs, so if we switch to postgresql dialect during parsing, current variable syntax will fail

**To Reproduce**

see #1099

**Expected behavior**
A clear and concise description of what you expected to happen.

**Additional context**
Add any other context about the problem here.

Start by reviewing issue #1099 and the PostgreSQL dialect parsing behavior described here. Determine the intended replacement for `@` and verify that variable syntax no longer conflicts with PostgreSQL's `@` operator during parsing.

Written by the indexing model from the issue text.