# Moose / Olap / Indexes Documentation – TypeScript

## Included Files
1. moose/olap/indexes/indexes.mdx

## Secondary Indexes

Source: moose/olap/indexes/indexes.mdx

Specifying indexes with Moose OLAP

## Indexes for ClickHouse tables

Moose lets you declare secondary/data-skipping indexes directly in your table definitions.
Moose generates the ClickHouse `INDEX` clauses on create and
plans `ALTER TABLE ADD/DROP INDEX` operations when you change them later.

### When to use indexes

- Use indexes to optimize selective predicates on large tables, especially string and high-cardinality columns.
- Common types: `minmax`, `Set(max_rows)`, `ngrambf_v1(...)`, `bloom_filter`.

### TypeScript

```ts

interface Events {
  id: string;
  user: string;
  message: string;
}

);
```

### Python

```python
from moose_lib.dmv2.olap_table import OlapTable, OlapConfig, MergeTreeEngine
from pydantic import BaseModel

class Events(BaseModel):
  id: str
  user: str
  message: str

events_table = OlapTable[Events](
  "Events",
  OlapConfig(
    engine=MergeTreeEngine(),
    order_by_fields=["id"],
    indexes=[
      OlapConfig.TableIndex(name="idx_user", expression="user", type="minmax", granularity=1),
      OlapConfig.TableIndex(name="idx_message_ngrams", expression="message", type="ngrambf_v1", arguments=["3","256","1","123"], granularity=1),
    ],
  ),
)
```

### How Moose applies changes

- On create, Moose emits `INDEX ...` entries inside `CREATE TABLE`.
- On change, Moose plans `ALTER TABLE DROP INDEX <name>` then `ADD INDEX ...` if the definition changed; pure adds/drops are applied as single operations.