index lookup query scan index may sent redundancy rpc when enable paging

[crazycs520]

1. Minimal reproduce step (Required)

drop table if exists t;
create table t (id int key auto_increment, b int, c int, index idx (b));
insert into t () values (), (), (), (), (), (), (), ();
insert into t (b) select b from t;
insert into t (b) select b from t;
insert into t (b) select b from t;
insert into t (b) select b from t;
insert into t (b) select b from t;
insert into t (b) select b from t;
insert into t (b) select b from t;
update t set b = id, c = id;

-- set following variables to default value. 
set @@tidb_enable_paging=1;
set @@tidb_min_paging_size=128;
set @@tidb_max_chunk_size=1024;

explain analyze select * from t use index(idx);

2. What did you see instead (Required)

The IndexFullScan_4 executor sends 3 cop RPC to scan index, but actually, 1 RPC is enough, because IndexLookUp_6 only has 1 table_task, which means IndexLookUp_6 neet to fetch initBatchSize which value will be 1024(from tidb_max_chunk_size=1024) index, then build a table task to execution. Enable paging is used to save memory, but in this case, enable paging with default tidb_min_paging_size is not a good idea, it will introduce redundancy RPC.

> explain analyze select * from t use index(idx) where b>0 and b < 1024;
+---------------------------+---------+---------+-----------+-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------+---------+------+
| id                        | estRows | actRows | task      | access object         | execution info                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       | operator info    | memory  | disk |
+---------------------------+---------+---------+-----------+-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------+---------+------+
| IndexLookUp_6             | 1024.00 | 1024    | root      |                       | time:5.93ms, loops:2, RU:3.923506, index_task: {total_time: 4.33ms, fetch_handle: 4.32ms, build: 1.04µs, wait: 10.7µs}, table_task: {total_time: 1.37ms, num: 1, concurrency: 5}, next: {wait_index: 4.5ms, wait_table_lookup_build: 123µs, wait_table_lookup_resp: 1.24ms}                                                                                                                                                                                                                                          |                  | 68.1 KB | N/A  |
| ├─IndexFullScan_4(Build)  | 1024.00 | 1024    | cop[tikv] | table:t, index:idx(b) | time:4.3ms, loops:2, cop_task: {num: 3, max: 2.61ms, min: 687.5µs, avg: 1.39ms, p95: 2.61ms, max_proc_keys: 480, p95_proc_keys: 480, tot_proc: 1.19ms, tot_wait: 208µs, copr_cache_hit_ratio: 0.00, build_task_duration: 53.2µs, max_distsql_concurrency: 1}, rpc_info:{Cop:{num_rpc:3, total_time:4.08ms}}, tikv_task:{proc max:0s, min:0s, avg: 0s, p80:0s, p95:0s, iters:11, tasks:3}, scan_detail: {total_process_keys: 1024, total_process_keys_size: 47104, total_keys: 3075, get_snapshot_time: 77.2µs, ro... | keep order:false | N/A     | N/A  |
| └─TableRowIDScan_5(Probe) | 1024.00 | 1024    | cop[tikv] | table:t               | time:1.21ms, loops:2, cop_task: {num: 1, max: 1.14ms, proc_keys: 1024, tot_proc: 716.8µs, tot_wait: 32µs, copr_cache_hit_ratio: 0.00, build_task_duration: 36.5µs, max_distsql_concurrency: 1}, rpc_info:{Cop:{num_rpc:1, total_time:1.13ms}}, tikv_task:{time:0s, loops:6}, scan_detail: {total_process_keys: 1024, total_process_keys_size: 43778, total_keys: 2049, get_snapshot_time: 11.5µs, rocksdb: {delete_skipped_count: 1024, key_skipped_count: 3072, block: {cache_hit_count: 2}}}, time_detail: {tot... | keep order:false | N/A     | N/A  |
+---------------------------+---------+---------+-----------+-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------+---------+------+

IndexFullScan_4(Build)  | 1024.00 |   ... rpc_info:{Cop:{num_rpc:3, ...

3. What did you expect to see?

IndexFullScan_4 only send 1 RPC.

IndexFullScan_4(Build)  | 1024.00 |   ... rpc_info:{Cop:{num_rpc:1, ...

4. What is your TiDB version? (Required)

nightly: a6e1a2e