add a tidb system variable tiflash_replica_read (pingcap#13806)

hfxsd · Aug 7, 2023 · 98a5537 · 98a5537
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diff --git a/system-variables.md b/system-variables.md
@@ -5081,6 +5081,31 @@ For details, see [Identify Slow Queries](/identify-slow-queries.md).
     * Integer greater than 0: the Fine Grained Shuffle feature is enabled. The window function pushed down to TiFlash is executed in multiple threads. The concurrency level is: min(`tiflash_fine_grained_shuffle_stream_count`, the number of physical threads on TiFlash nodes).
 - Theoretically, the performance of the window function increases linearly with this value. However, if the value exceeds the actual number of physical threads, it instead leads to performance degradation.
 
+### tiflash_replica_read <span class="version-mark">New in v7.3.0</span>
+
+> **Note:**
+>
+> This TiDB variable is not applicable to TiDB Cloud.
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Type: Enumeration
+- Default value: `all_replicas`
+- Value options: `all_replicas`, `closest_adaptive`, or `closest_replicas`
+- This variable is used to set the strategy for selecting TiFlash replicas when a query requires the TiFlash engine.
+    - `all_replicas` means using all available TiFlash replicas for analytical computing.
+    - `closest_adaptive` means preferring to use TiFlash replicas in the same zone as the TiDB node initiating the query. If replicas in this zone do not contain all the required data, the query will involve TiFlash replicas from other zones along with their corresponding TiFlash nodes.
+    - `closest_replicas` means using only TiFlash replicas in the same zone as the TiDB node initiating the query. If replicas in this zone do not contain all the required data, the query will return an error.
+
+<CustomContent platform="tidb">
+
+> **Note:**
+> 
+> - If TiDB nodes do not have [zone attributes](/schedule-replicas-by-topology-labels.md#optional-configure-labels-for-tidb) configured and `tiflash_replica_read` is not set to `all_replicas`, TiFlash ignores the replica selection strategy. Instead, it uses all TiFlash replicas for queries and returns the `The variable tiflash_replica_read is ignored.` warning.
+> - If TiFlash nodes do not have [zone attributes](/schedule-replicas-by-topology-labels.md#configure-labels-for-tikv-and-tiflash) configured, they are treated as nodes not belonging to any zone.
+
+</CustomContent>
+
 ### time_zone
 
 - Scope: SESSION | GLOBAL

diff --git a/tiflash/create-tiflash-replicas.md b/tiflash/create-tiflash-replicas.md
@@ -256,4 +256,6 @@ When configuring replicas, if you need to distribute TiFlash replicas to multipl
 
 For more information about scheduling replicas by using labels, see [Schedule Replicas by Topology Labels](/schedule-replicas-by-topology-labels.md), [Multiple Data Centers in One City Deployment](/multi-data-centers-in-one-city-deployment.md), and [Three Data Centers in Two Cities Deployment](/three-data-centers-in-two-cities-deployment.md).
 
+TiFlash supports configuring the replica selection strategy for different zones. For more information, see [`tiflash_replica_read`](/system-variables.md#tiflash_replica_read-new-in-v730).
+
 </CustomContent>
Original file line number	Diff line number	Diff line change
Expand Up		@@ -256,4 +256,6 @@ When configuring replicas, if you need to distribute TiFlash replicas to multipl

		For more information about scheduling replicas by using labels, see [Schedule Replicas by Topology Labels](/schedule-replicas-by-topology-labels.md), [Multiple Data Centers in One City Deployment](/multi-data-centers-in-one-city-deployment.md), and [Three Data Centers in Two Cities Deployment](/three-data-centers-in-two-cities-deployment.md).

		TiFlash supports configuring the replica selection strategy for different zones. For more information, see [`tiflash_replica_read`](/system-variables.md#tiflash_replica_read-new-in-v730).

		</CustomContent>