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In addition to indexing columns, Postgres expressions can also be indexed. For instance, the following statement indexes an expression which concatenates description and category: 
CREATE INDEX search_idx ON mock_items
USING bm25 (id, ((description || ' ' || category)::pdb.simple('alias=description_concat')))
WITH (key_field='id');
To index an expression:
  1. Add the expression to the column list. In this example, the expression is description || ' ' || category.
  2. Cast it to a tokenizer, in this example pdb.simple.
  3. ParadeDB will try and infer a field name based on the field used in the expression. However, if the field name cannot be inferred (e.g. because the expression involves more than one field), you will be required to add an alias=<alias_name> to the tokenizer.
Querying against the expression is the same as querying a regular field: 
SELECT description, rating, category
FROM mock_items
WHERE (description || ' ' || category) &&& 'running shoes';
The expression on the left-hand side of the operator must exactly match the expression that was indexed.
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