---
title: Indexing Expressions
---

In addition to indexing columns, Postgres expressions can also be indexed.
For instance, the following statement indexes an expression which concatenates `description` and `category`:

```sql
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](/v2/tokenizers), 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:

```sql
SELECT description, rating, category
FROM mock_items
WHERE (description || ' ' || category) &&& 'running shoes';
```

<Note>
  The expression on the left-hand side of the operator must exactly match the
  expression that was indexed.
</Note>