---
title: Reindexing
description: Rebuild an existing index with zero downtime
canonical: https://docs.paradedb.com/documentation/indexing/reindexing
---

## Changing the Schema

If an index's schema is changed, it must be rebuilt. This includes:

1. Adding a field to the index
2. Removing a field from the index
3. Renaming an indexed column in the underlying table
4. Changing a field's tokenizer

Let's assume the existing index is called `search_idx`, and we want to create a new index called `search_idx_v2`.
First, use `CREATE INDEX CONCURRENTLY` to build a new index in the background.

<CodeGroup>
```sql SQL
CREATE INDEX CONCURRENTLY search_idx_v2
ON mock_items USING bm25 (id, description, category)
WITH (key_field = 'id');
```

```python Django
from django.db import connection
from paradedb.indexes import BM25Index

with connection.schema_editor(atomic=False) as schema_editor:
    schema_editor.add_index(
        MockItem,
        BM25Index(
            fields={
                "id": {},
                "description": {},
                "category": {},
            },
            key_field="id",
            name="search_idx_v2",
        ),
        concurrently=True,
    )
```

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

idx = Index(
    "search_idx_v2",
    indexing.BM25Field(MockItem.id),
    indexing.BM25Field(MockItem.description),
    indexing.BM25Field(MockItem.category),
    postgresql_using="bm25",
    postgresql_with={"key_field": "id"},
    postgresql_concurrently=True,
)

with engine.connect() as conn:
    conn = conn.execution_options(isolation_level="AUTOCOMMIT")
    idx.create(conn)
```

```ruby Rails
ActiveRecord::Base.connection.add_bm25_index(
  :mock_items,
  fields: {
    id: {},
    description: {},
    category: {}
  },
  key_field: :id,
  name: :search_idx_v2,
  concurrently: true
)
```

</CodeGroup>

<Note>
  The `CONCURRENTLY` clause is required. `CONCURRENTLY` allows the existing
  index to continue serving queries while the new index is being built.
</Note>

From another session, you can use `pg_stat_progress_create_index` to [track the progress](/documentation/indexing/create-index#track-create-index-progress) of the new index.

Once the new index is done building, confirm that it is valid:

```sql
SELECT ix.indisvalid, ix.indisready, ix.indislive
FROM pg_class i
JOIN pg_index ix ON ix.indexrelid = i.oid
WHERE i.relname = 'search_idx_v2';
```

```csv Expected Response
 indisvalid | indisready | indislive
------------+------------+-----------
 t          | t          | t
(1 row)
```

If all three columns are `true`, the original index can safely be dropped, which will redirect queries to the new index.

```sql
DROP INDEX search_idx;
```

## Rebuilding the Index

`REINDEX` is used to rebuild an index without changing the schema.
The basic syntax for `REINDEX` is:

```sql
REINDEX INDEX search_idx;
```

This operation takes an exclusive lock on the table, which blocks incoming writes (but not reads) while the new index is being built.

To allow for concurrent writes during a reindex, use `REINDEX CONCURRENTLY`:

```sql
REINDEX INDEX CONCURRENTLY search_idx;
```

The tradeoff is that `REINDEX CONCURRENTLY` is slower than a plain `REINDEX`. Generally speaking, `REINDEX CONCURRENTLY` is recommended for
production systems that cannot tolerate temporarily blocked writes.

## Important Caveats

Although `CREATE INDEX CONCURRENTLY` and `REINDEX CONURRENTLY` run in the background, Postgres requires that the
session that is executing the command remain open. If the session is closed,
Postgres will cancel the operation. This is relevant if you are using a
connection pooler like `pgbouncer`, which may terminate
sessions after a certain idle timeout is reached.

If `REINDEX CONCURRENTLY` fails or is cancelled, an invalid transient index will be left behind that must be dropped manually.
To check for invalid indexes in `psql`, run `\d <table_name>` and look for `INVALID` indexes.
