Support hash-sharded indexes that hash only a prefix of the index key columns

src/current/_includes/v26.2/misc/index-storage-parameters.md

@@ -1,6 +1,7 @@
 | Parameter name | Description | Data type | Default value
 |---------------------+----------------------|-----|------|
 | `bucket_count` | The number of buckets into which a [hash-sharded index]({% link {{ page.version.version }}/hash-sharded-indexes.md %}) will split. | Integer | The value of the `sql.defaults.default_hash_sharded_index_bucket_count` [cluster setting]({% link {{ page.version.version }}/cluster-settings.md %}). |
+| <a name="storage-parameter-shard-columns"></a> `shard_columns` | The first key column or columns to use when computing the shard column for a [hash-sharded index]({% link {{ page.version.version }}/hash-sharded-indexes.md %}). The value must match a prefix of the index key columns. If this parameter is omitted, CockroachDB hashes all index key columns. | Tuple of column identifiers or a single column identifier | All index key columns |
 | `geometry_max_x` | The maximum X-value of the [spatial reference system]({% link {{ page.version.version }}/architecture/glossary.md %}#spatial-reference-system) for the object(s) being covered. This only needs to be set if you are using a custom [SRID]({% link {{ page.version.version }}/architecture/glossary.md %}#srid). | | Derived from SRID bounds, else `(1 << 31) -1`. |
 | `geometry_max_y` | The maximum Y-value of the [spatial reference system]({% link {{ page.version.version }}/architecture/glossary.md %}#spatial-reference-system) for the object(s) being covered. This only needs to be set if you are using a custom [SRID]({% link {{ page.version.version }}/architecture/glossary.md %}#srid). | | Derived from SRID bounds, else `(1 << 31) -1`. |
 | `geometry_min_x` | The minimum X-value of the [spatial reference system]({% link {{ page.version.version }}/architecture/glossary.md %}#spatial-reference-system) for the object(s) being covered. This only needs to be set if the default bounds of the SRID are too large/small for the given data, or SRID = 0 and you wish to use a smaller range (unfortunately this is currently not exposed, but is viewable on <https://epsg.io/3857>). By default, SRID = 0 assumes `[-min int32, max int32]` ranges. | | Derived from SRID bounds, else `-(1 << 31)`. |

src/current/v26.2/hash-sharded-indexes.md

@@ -43,6 +43,20 @@ We recommend doing thorough performance testing of your workload with different
 
 {% include {{page.version.version}}/sql/sql-defaults-cluster-settings-deprecation-notice.md %}
 
+### Shard columns
+
+By default, CockroachDB hashes all key columns in a hash-sharded index when it computes the shard column.
+
+If queries usually constrain only the first key column or columns of a multi-column index, set the [`shard_columns` storage parameter]({% link {{ page.version.version }}/with-storage-parameter.md %}#storage-parameter-shard-columns) to hash only that prefix of the index key columns. The value of `shard_columns` must match a prefix of the index key columns.
+
+This is useful when queries filter on the leading index columns but still need to scan or sort by later index columns within each shard. For example, the following statement hashes only `region`, while keeping `city` and `population` as ordered index columns:
+
+{% include_cached copy-clipboard.html %}
+~~~ sql
+> CREATE INDEX idx_region_city ON data_test (region, city, population)
+    USING HASH WITH (bucket_count = 4, shard_columns = (region));
+~~~
+
 ### Hash-sharded indexes on partitioned tables
 
 You can create hash-sharded indexes with implicit partitioning under the following scenarios: