# Index a filtered query

This example adds an index for a category predicate and checks that the selected rows remain the same. The small dataset illustrates the workflow; its timing is not a performance claim.

## Prepare and query the data

```sql
CREATE TABLE logs(id INTEGER PRIMARY KEY, category String);
INSERT INTO logs VALUES (1, 'read'), (2, 'write'), (3, 'read');
```

```sql
SELECT id
FROM logs
WHERE category = 'read'
ORDER BY id
FORMAT csv;
```

```text
1
3
```

## Add an index

```sql
CREATE INDEX logs_category ON logs(category);
ANALYZE;
```

Inspect EXPLAIN QUERY PLAN for the predicate. The selected plan depends on the data and implementation version; an index is not a universal speedup.

## Verify the result again

```sql
SELECT id
FROM logs
WHERE category = 'read'
ORDER BY id
FORMAT csv;
```

```text
1
3
```

Compare measurements only after confirming the result. See [query measurements](https://coordinatordb.com/docs/guides/clickhouse/performance-and-monitoring/isolate-query-bottlenecks) and [`CREATE INDEX`](https://coordinatordb.com/docs/reference/statements/create/create-index).