update 25.1 session vars (#19371)

* update session vars for 25.1

src/current/_includes/v25.1/misc/session-vars.md

@@ -1,5 +1,6 @@
 | Variable name | Description | Initial value | Modify with [`SET`]({% link {{ page.version.version }}/set-vars.md %})? | View with [`SHOW`]({% link {{ page.version.version }}/show-vars.md %})? |
 |---|---|---|---|---|
+| <a id="always-distribute-full-scans"></a> `always_distribute_full_scans` | **New in v25.1:** When set to `on`, full table scans are always [distributed]({% link {{ page.version.version }}/architecture/sql-layer.md %}#distsql). | `off` | Yes | Yes |
 | <a id="application-name"></a> `application_name` | The current application name for statistics collection. | Empty string, or `cockroach` for sessions from the [built-in SQL client]({% link {{ page.version.version }}/cockroach-sql.md %}). | Yes | Yes |
 | <a id="autocommit-before-ddl"></a> `autocommit_before_ddl` | When the [`autocommit_before_ddl` session setting]({% link {{page.version.version}}/set-vars.md %}#autocommit-before-ddl) is set to `on`, any schema change statement that is sent during an [explicit transaction]({% link {{page.version.version}}/transactions.md %}) will cause the transaction to [commit]({% link {{page.version.version}}/commit-transaction.md %}) before executing the schema change. This is useful because [CockroachDB does not fully support multiple schema changes in a single transaction]({% link {{ page.version.version }}/online-schema-changes.md %}#schema-changes-within-transactions). **New in v25.1**: This setting is enabled by default. To disable it for all [roles]({% link {{ page.version.version }}/alter-role.md %}), issue the following statement: `ALTER ROLE ALL SET autocommit_before_ddl = false`  | `on` | Yes | Yes |
 | <a id="bytea-output"></a> `bytea_output` | The [mode for conversions from `STRING` to `BYTES`]({% link {{ page.version.version }}/bytes.md %}#supported-conversions). | hex | Yes | Yes |
@@ -18,12 +19,16 @@
 | <a id="default-transaction-read-only"></a> `default_transaction_read_only` | The default transaction access mode for the current session. <br/>If set to `on`, only read operations are allowed in transactions in the current session; if set to `off`, both read and write operations are allowed. See [`SET TRANSACTION`]({% link {{ page.version.version }}/set-transaction.md %}) for more details. | `off` | Yes | Yes |
 | <a id="default-transaction-use-follower-reads"></a> `default_transaction_use_follower_reads` | If set to on, all read-only transactions use [`AS OF SYSTEM TIME follower_read_timestamp()`]({% link {{ page.version.version }}/as-of-system-time.md %}) to allow the transaction to use follower reads. <br/>If set to `off`, read-only transactions will only use follower reads if an `AS OF SYSTEM TIME` clause is specified in the statement, with an interval of at least 4.8 seconds. | `off` | Yes | Yes |
 | <a id="disable-changefeed-replication"></a> `disable_changefeed_replication` | When `true`, [changefeeds]({% link {{ page.version.version }}/changefeed-messages.md %}#filtering-changefeed-messages) will not emit messages for any changes (e.g., `INSERT`, `UPDATE`) issued to watched tables during that session. | `false` | Yes | Yes |
-| <a id="disallow-full-table-scans"></a> `disallow_full_table_scans` | If set to `on`, queries on "large" tables with a row count greater than [`large_full_scan_rows`](#large-full-scan-rows) will not use full table or index scans. If no other query plan is possible, queries will return an error message. This setting does not apply to internal queries, which may plan full table or index scans without checking the session variable. | `off` | Yes | Yes || <a id="distsql"></a> `distsql` | The query distribution mode for the session. By default, CockroachDB determines which queries are faster to execute if distributed across multiple nodes, and all other queries are run through the gateway node. | `auto` | Yes | Yes |
+| <a id="disallow-full-table-scans"></a> `disallow_full_table_scans` | If set to `on`, queries on "large" tables with a row count greater than [`large_full_scan_rows`](#large-full-scan-rows) will not use full table or index scans. If no other query plan is possible, queries will return an error message. This setting does not apply to internal queries, which may plan full table or index scans without checking the session variable. | `off` | Yes | Yes |
+| <a id="distribute-group-by-row-count-threshold"></a> `distribute_group_by_row_count_threshold` | **New in v25.1:** Minimum number of rows that a `GROUP BY` operation must process in order to be [distributed]({% link {{ page.version.version }}/architecture/sql-layer.md %}#distsql). | `1000` | Yes | Yes |
+| <a id="distribute-scan-row-count-threshold"></a> `distribute_scan_row_count_threshold` | **New in v25.1:** Minimum number of rows that a scan operation must process in order to be [distributed]({% link {{ page.version.version }}/architecture/sql-layer.md %}#distsql). This means that full table scans will not be distributed if they read fewer than this number of rows. To always distribute full table scans, set [`always_distribute_full_scans`](#always-distribute-full-scans). | `10000` | Yes | Yes |
+| <a id="distribute-sort-row-count-threshold"></a> `distribute_sort_row_count_threshold` | **New in v25.1:** Minimum number of rows that a sort operation must process in order to be [distributed]({% link {{ page.version.version }}/architecture/sql-layer.md %}#distsql). | `1000` | Yes | Yes |
+| <a id="distsql"></a> `distsql` | The query distribution mode for the session. By default, CockroachDB determines which queries are faster to execute if distributed across multiple nodes. Distribution preferences for `GROUP BY`, scan, and sort operations are set with [`distribute_group_by_row_count_threshold`](#distribute-group-by-row-count-threshold), [`distribute_scan_row_count_threshold.`](#distribute-scan-row-count-threshold) and [`distribute_sort_row_count_threshold.`](#distribute-sort-row-count-threshold), respectively. All other queries are run through the gateway node. | `auto` | Yes | Yes |
 | <a id="enable-auto-rehoming"></a> `enable_auto_rehoming` | When enabled, the [home regions]({% link {{ page.version.version }}/alter-table.md %}#crdb_region) of rows in [`REGIONAL BY ROW`]({% link {{ page.version.version }}/alter-table.md %}#set-the-table-locality-to-regional-by-row) tables are automatically set to the region of the [gateway node]({% link {{ page.version.version }}/ui-sessions-page.md %}#session-details-gateway-node) from which any [`UPDATE`]({% link {{ page.version.version }}/update.md %}) or [`UPSERT`]({% link {{ page.version.version }}/upsert.md %}) statements that operate on those rows originate. | `off` | Yes | Yes |
 | <a id="enable-durable-locking-for-serializable"></a> `enable_durable_locking_for_serializable` | Indicates whether CockroachDB replicates [`FOR UPDATE` and `FOR SHARE`]({% link {{ page.version.version }}/select-for-update.md %}#lock-strengths) locks via [Raft]({% link {{ page.version.version }}/architecture/replication-layer.md %}#raft), allowing locks to be preserved when leases are transferred. Note that replicating `FOR UPDATE` and `FOR SHARE` locks will add latency to those statements. This setting only affects `SERIALIZABLE` transactions and matches the default `READ COMMITTED` behavior when enabled. | `off` | Yes | Yes |
 | <a id="enable-experimental-alter-column-type-general"></a> `enable_experimental_alter_column_type_general` | If `on`, it is possible to [alter column data types]({% link {{ page.version.version }}/alter-table.md %}#alter-column-data-types). | `off` | Yes | Yes |
 | <a id="enable-implicit-fk-locking-for-serializable"></a> `enable_implicit_fk_locking_for_serializable` | Indicates whether CockroachDB uses [shared locks]({% link {{ page.version.version }}/select-for-update.md %}#lock-strengths) to perform [foreign key]({% link {{ page.version.version }}/foreign-key.md %}) checks. To take effect, the [`enable_shared_locking_for_serializable`](#enable-shared-locking-for-serializable) setting must also be enabled. This setting only affects `SERIALIZABLE` transactions and matches the default `READ COMMITTED` behavior when enabled. | `off` | Yes | Yes |
-| <a id="enable-implicit-select-for-update"></a> `enable_implicit_select_for_update` | Indicates whether [`UPDATE`]({% link {{ page.version.version }}/update.md %}) and [`UPSERT`]({% link {{ page.version.version }}/upsert.md %}) statements acquire locks using the `FOR UPDATE` locking mode during their initial row scan, which improves performance for contended workloads.<br/><br/>For more information about how `FOR UPDATE` locking works, see the documentation for [`SELECT FOR UPDATE`]({% link {{ page.version.version }}/select-for-update.md %}). | `on` | Yes | Yes |
+| <a id="enable-implicit-select-for-update"></a> `enable_implicit_select_for_update` | Indicates whether [`UPDATE`]({% link {{ page.version.version }}/update.md %}), [`UPSERT`]({% link {{ page.version.version }}/upsert.md %}), and [`DELETE`]({% link {{ page.version.version }}/delete.md %}) statements acquire locks using the `FOR UPDATE` locking mode during their initial row scan, which improves performance for contended workloads.<br/><br/>For more information about how `FOR UPDATE` locking works, see the documentation for [`SELECT FOR UPDATE`]({% link {{ page.version.version }}/select-for-update.md %}). | `on` | Yes | Yes |
 | <a id="enable-implicit-transaction-for-batch-statements"></a> `enable_implicit_transaction_for_batch_statements` | Indicates whether multiple statements in a single query (a "batch statement") will all run in the same implicit transaction, which matches the PostgreSQL wire protocol. | `on` | Yes | Yes |
 | <a id="enable-insert-fast-path"></a> `enable_insert_fast_path` | Indicates whether CockroachDB will use a specialized execution operator for inserting into a table. We recommend leaving this setting `on`. | `on` | Yes | Yes |
 | <a id="enable-shared-locking-for-serializable"></a> `enable_shared_locking_for_serializable` | Indicates whether [shared locks]({% link {{ page.version.version }}/select-for-update.md %}#lock-strengths) are enabled for `SERIALIZABLE` transactions. When `off`, `SELECT` statements using `FOR SHARE` are still permitted under `SERIALIZABLE` isolation, but silently do not lock. | `off` | Yes | Yes |