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docs: Add supported index key types to Index docs page #4606

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30 changes: 30 additions & 0 deletions docs/docs/00200-core-concepts/00300-tables/00300-indexes.md
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Expand Up @@ -31,6 +31,36 @@ SpacetimeDB supports two index types:
| B-tree | General purpose | Any | Yes |
| Direct | Dense integer sequences | `u8`, `u16`, `u32`, `u64` | No |

### Supported Column Types

Not all column types can be used as index keys. The following types are supported for B-tree indexes:

| Category | Types |
|----------|-------|
| Integers | `u8`, `u16`, `u32`, `u64`, `u128`, `u256`, `i8`, `i16`, `i32`, `i64`, `i128`, `i256` |
| Boolean | `bool` |
| Strings | `String` |
| Identifiers | `Identity`, `ConnectionId`, `Uuid`, `Hash` |
| Enums | No-payload (C-style) enums annotated with `#[derive(SpacetimeType)]` |

The following types are **not** supported as index keys:

| Type | Reason |
|------|--------|
| `f32`, `f64` | Floating-point values do not have a total ordering (`NaN` is not comparable) |
| `ScheduleAt`, `TimeDuration`, `Timestamp` | Not yet supported ([#2650](https://github.com/clockworklabs/SpacetimeDB/issues/2650)) |
| `Vec<T>`, arrays | Variable-length collections are not indexable |
| Enums with payloads | Only no-payload (C-style) enums are supported |
| Nested structs | Product types cannot be used as index keys |

If you attempt to use an unsupported type as an index key, you will get a compile error. For multi-column indexes, every column in the index must use a supported type.

:::tip Workaround for floating-point data
If you need to index floating-point coordinates (for example, `x` and `y` positions), consider storing them as scaled integers. For instance, multiply by 1000 and store as `i32` to get three decimal places of precision while remaining indexable.
:::

Direct indexes have additional restrictions: only `u8`, `u16`, `u32`, `u64`, and no-payload enums are supported.

### B-tree Indexes

B-trees maintain data in sorted order, enabling both equality lookups (`x = 5`) and range queries (`x > 5`, `x BETWEEN 1 AND 10`). The sorted structure also supports prefix matching on multi-column indexes. B-tree is the default and most commonly used index type.
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