---
title: Indexing Expressions
description: Add Postgres expressions to the index
canonical: https://docs.paradedb.com/documentation/indexing/indexing-expressions
---

In addition to indexing columns, Postgres expressions can also be indexed.

## Indexing Text/JSON Expressions

The following statement indexes an expression which concatenates `description` and `category`, which are both text fields:

<CodeGroup>
```sql SQL
CREATE INDEX search_idx ON mock_items
USING bm25 (id, ((description || ' ' || category)::pdb.simple('alias=description_concat')))
WITH (key_field='id');
```

```python Django
from django.db import connection, models
from django.db.models import F, Func, Value
from paradedb.indexes import BM25Index, IndexExpression

with connection.schema_editor() as schema_editor:
    schema_editor.add_index(
        MockItem,
        BM25Index(
            fields={"id": {}},
            expressions=[
                IndexExpression(
                    Func(
                        F("description"),
                        Value(" "),
                        F("category"),
                        template="(%(expressions)s)",
                        arg_joiner=" || ",
                        output_field=models.TextField(),
                    ),
                    alias="description_concat",
                    tokenizer="simple",
                ),
            ],
            key_field="id",
            name="search_idx",
        ),
    )
```

```python SQLAlchemy
from sqlalchemy import Index
from paradedb.sqlalchemy import indexing

idx = Index(
    "search_idx",
    indexing.BM25Field(MockItem.id),
    indexing.BM25Field(
        MockItem.description + " " + MockItem.category,
        tokenizer=indexing.tokenize.simple(alias="description_concat"),
    ),
    postgresql_using="bm25",
    postgresql_with={"key_field": "id"},
)

with engine.begin() as conn:
    idx.create(conn)
```

```ruby Rails
ActiveRecord::Base.connection.add_bm25_index(
  :mock_items,
  fields: {
    id: {},
    "(description || ' ' || category)" => {
      tokenizer: :simple,
      alias: "description_concat"
    }
  },
  key_field: :id,
  name: :search_idx
)
```

</CodeGroup>

To index a text/JSON expression:

1. Add the expression to the column list. In this example, the expression is `description || ' ' || category`.
2. Cast it to a [tokenizer](/documentation/tokenizers/overview), 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>

## Indexing Non-Text Expressions

To index a non-text expression, cast the expression to `pdb.alias`. For example, the following statement indexes
the expression `rating + 1`, which returns an integer:

<CodeGroup>
```sql SQL
CREATE INDEX search_idx ON mock_items
USING bm25 (id, description, ((rating + 1)::pdb.alias('rating')))
WITH (key_field='id');
```

```python Django
from django.db import connection
from django.db.models import F
from paradedb.indexes import BM25Index, IndexExpression

with connection.schema_editor() as schema_editor:
    schema_editor.add_index(
        MockItem,
        BM25Index(
            fields={"id": {}, "description": {}},
            expressions=[
                IndexExpression(
                    F("rating") + 1,
                    alias="rating",
                ),
            ],
            key_field="id",
            name="search_idx",
        ),
    )
```

```python SQLAlchemy
from sqlalchemy import Index
from paradedb.sqlalchemy import indexing, pdb

idx = Index(
    "search_idx",
    indexing.BM25Field(MockItem.id),
    indexing.BM25Field(MockItem.description),
    indexing.BM25Field(
        pdb.alias(MockItem.rating + 1, "rating"),
    ),
    postgresql_using="bm25",
    postgresql_with={"key_field": "id"},
)

with engine.begin() as conn:
    idx.create(conn)
```

```ruby Rails
ActiveRecord::Base.connection.add_bm25_index(
  :mock_items,
  fields: {
    id: {},
    description: {},
    "(rating + 1)" => { alias: "rating" }
  },
  key_field: :id,
  name: :search_idx
)
```

</CodeGroup>

With the expression indexed, queries containing the expression can be pushed down to the ParadeDB index:

```sql
SELECT description, rating, category
FROM mock_items
WHERE description &&& 'running shoes'
AND rating + 1 > 3;
```
