Zone Map Index¶
Zone maps are a columnar database technique for predicate pushdown and scan pruning. They break data into fixed-size chunks called "zones" and maintain summary statistics (min, max, null count) for each zone, enabling efficient filtering by eliminating zones that cannot contain matching values.
Zone maps are "inexact" filters - they can definitively exclude zones but may include false positives that require rechecking.
Index Details¶
Storage Layout¶
The zone map index stores zone statistics in a single file:
zonemap.lance- Zone statistics for query pruning
Zone Statistics File Schema¶
| Column | Type | Nullable | Description |
|---|---|---|---|
min |
{DataType} | true | Minimum value in the zone |
max |
{DataType} | true | Maximum value in the zone |
null_count |
UInt32 | false | Number of null values in the zone |
nan_count |
UInt32 | false | Number of NaN values (for float types) |
fragment_id |
UInt64 | false | Fragment containing this zone |
zone_start |
UInt64 | false | Starting row offset within the fragment |
zone_length |
UInt32 | false | Number of rows in this zone |
Schema Metadata¶
| Key | Type | Description |
|---|---|---|
rows_per_zone |
String | Number of rows per zone (default: "8192") |
Accelerated Queries¶
The zone map index provides inexact results for the following query types:
| Query Type | Description | Operation | Result Type |
|---|---|---|---|
| Equals | column = value |
Includes zones where min ≤ value ≤ max | AtMost |
| Range | column BETWEEN a AND b |
Includes zones where ranges overlap | AtMost |
| IsIn | column IN (v1, v2, ...) |
Includes zones that could contain any value | AtMost |
| IsNull | column IS NULL |
Includes zones where null_count > 0 | AtMost |