Clarify ReplicaReadMixed also uses learner replicas (#22170) (#22183)

Author: ti-chi-bot

follower-read.md

@@ -33,9 +33,9 @@ This variable is used to set the expected data read mode.
 
 - When you set the value of `tidb_replica_read` to `leader` or an empty string, TiDB maintains its default behavior and sends all read operations to the leader replica to perform.
 - When you set the value of `tidb_replica_read` to `follower`, TiDB selects a follower replica of the Region to perform read operations. If the Region has learner replicas, TiDB also considers them for reads with the same priority. If no available follower or learner replicas exist for the current Region, TiDB reads from the leader replica.
-- When the value of `tidb_replica_read` is set to `leader-and-follower`, TiDB can select any replicas to perform read operations. In this mode, read requests are load balanced between the leader and follower.
+- When the value of `tidb_replica_read` is set to `leader-and-follower`, TiDB can select any replicas to perform read operations. In this mode, read requests are load balanced between the leader, follower, and learner.
 - When the value of `tidb_replica_read` is set to `prefer-leader`, TiDB prefers to select the leader replica to perform read operations. If the leader replica is obviously slow in processing read operations (such as caused by disk or network performance jitter), TiDB will select other available follower replicas to perform read operations.
-- When the value of `tidb_replica_read` is set to `closest-replicas`, TiDB prefers to select a replica in the same availability zone to perform read operations, which can be a leader or a follower. If there is no replica in the same availability zone, TiDB reads from the leader replica.
+- When the value of `tidb_replica_read` is set to `closest-replicas`, TiDB prefers to select a replica in the same availability zone to perform read operations, which can be a leader, a follower, or a learner. If there is no replica in the same availability zone, TiDB reads from the leader replica.
 - When the value of `tidb_replica_read` is set to `closest-adaptive`:
 
     - If the estimated result of a read request is greater than or equal to the value of [`tidb_adaptive_closest_read_threshold`](/system-variables.md#tidb_adaptive_closest_read_threshold-new-in-v630), TiDB prefers to select a replica in the same availability zone for read operations. To avoid unbalanced distribution of read traffic across availability zones, TiDB dynamically detects the distribution of availability zones for all online TiDB and TiKV nodes. In each availability zone, the number of TiDB nodes whose `closest-adaptive` configuration takes effect is limited, which is always the same as the number of TiDB nodes in the availability zone with the fewest TiDB nodes, and the other TiDB nodes automatically read from the leader replica. For example, if TiDB nodes are distributed across 3 availability zones (A, B, and C), where A and B each contains 3 TiDB nodes and C contains only 2 TiDB nodes, the number of TiDB nodes whose `closest-adaptive` configuration takes effect in each availability zone is 2, and the other TiDB node in each of the A and B availability zones automatically selects the leader replica for read operations.