*: implement IterReverse for tikvSnapshot and use desc scan to get latest N ddl history jobs.

[crazycs520]

What problem does this PR solve?

Currently admin show ddl jobs get all ddl history jobs, but we only need the latest N ddl history jobs. So we can use desc scan to get latest N ddl history jobs.

What is changed and how it works?

1. Implement IterReverse for tikvSnapshot.
2. Use IterReverse to get latest ddl history jobs for admin show ddl jobs

Compare
There are 17083 ddl history jobs.

mysql root@localhost:test> admin show ddl jobs 100000
17083 rows in set
Time: 2.323s

Before spend 0.5s.

mysql root@localhost:test> admin show ddl jobs 10
+--------+---------+------------+--------------+--------------+-----------+----------+-----------+-----------------------------------+--------+
| JOB_ID | DB_NAME | TABLE_NAME | JOB_TYPE     | SCHEMA_STATE | SCHEMA_ID | TABLE_ID | ROW_COUNT | START_TIME                        | STATE  |
+--------+---------+------------+--------------+--------------+-----------+----------+-----------+-----------------------------------+--------+
| 34165  | test23  | t2317      | create table | public       | 15586     | 34164    | 0         | 2019-04-15 19:44:45.005 +0800 CST | synced |
| 34163  | test22  | t2340      | create table | public       | 15456     | 34162    | 0         | 2019-04-15 19:44:44.955 +0800 CST | synced |
| 34161  | test20  | t2421      | create table | public       | 15238     | 34160    | 0         | 2019-04-15 19:44:44.854 +0800 CST | synced |
| 34159  | test21  | t2382      | create table | public       | 15310     | 34158    | 0         | 2019-04-15 19:44:44.754 +0800 CST | synced |
| 34157  | test23  | t2316      | create table | public       | 15586     | 34156    | 0         | 2019-04-15 19:44:44.654 +0800 CST | synced |
| 34155  | test22  | t2339      | create table | public       | 15456     | 34154    | 0         | 2019-04-15 19:44:44.458 +0800 CST | synced |
| 34153  | test20  | t2420      | create table | public       | 15238     | 34152    | 0         | 2019-04-15 19:44:43.854 +0800 CST | synced |
| 34151  | test21  | t2381      | create table | public       | 15310     | 34150    | 0         | 2019-04-15 19:44:43.706 +0800 CST | synced |
| 34149  | test23  | t2315      | create table | public       | 15586     | 34148    | 0         | 2019-04-15 19:44:43.654 +0800 CST | synced |
| 34147  | test22  | t2338      | create table | public       | 15456     | 34146    | 0         | 2019-04-15 19:44:43.506 +0800 CST | synced |
+--------+---------+------------+--------------+--------------+-----------+----------+-----------+-----------------------------------+--------+
10 rows in set
Time: 0.517s

In This PR spend 0.01s.

mysql root@localhost:test> admin show ddl jobs
+--------+---------+------------+--------------+--------------+-----------+----------+-----------+-----------------------------------+--------+
| JOB_ID | DB_NAME | TABLE_NAME | JOB_TYPE     | SCHEMA_STATE | SCHEMA_ID | TABLE_ID | ROW_COUNT | START_TIME                        | STATE  |
+--------+---------+------------+--------------+--------------+-----------+----------+-----------+-----------------------------------+--------+
| 34165  | test23  | t2317      | create table | public       | 15586     | 34164    | 0         | 2019-04-15 19:44:45.005 +0800 CST | synced |
| 34163  | test22  | t2340      | create table | public       | 15456     | 34162    | 0         | 2019-04-15 19:44:44.955 +0800 CST | synced |
| 34161  | test20  | t2421      | create table | public       | 15238     | 34160    | 0         | 2019-04-15 19:44:44.854 +0800 CST | synced |
| 34159  | test21  | t2382      | create table | public       | 15310     | 34158    | 0         | 2019-04-15 19:44:44.754 +0800 CST | synced |
| 34157  | test23  | t2316      | create table | public       | 15586     | 34156    | 0         | 2019-04-15 19:44:44.654 +0800 CST | synced |
| 34155  | test22  | t2339      | create table | public       | 15456     | 34154    | 0         | 2019-04-15 19:44:44.458 +0800 CST | synced |
| 34153  | test20  | t2420      | create table | public       | 15238     | 34152    | 0         | 2019-04-15 19:44:43.854 +0800 CST | synced |
| 34151  | test21  | t2381      | create table | public       | 15310     | 34150    | 0         | 2019-04-15 19:44:43.706 +0800 CST | synced |
| 34149  | test23  | t2315      | create table | public       | 15586     | 34148    | 0         | 2019-04-15 19:44:43.654 +0800 CST | synced |
| 34147  | test22  | t2338      | create table | public       | 15456     | 34146    | 0         | 2019-04-15 19:44:43.506 +0800 CST | synced |
+--------+---------+------------+--------------+--------------+-----------+----------+-----------+-----------------------------------+--------+
10 rows in set
Time: 0.010s

Check List

Tests

• Unit test

Code changes

Side effects

Related changes

[crazycs520]

@MyonKeminta @tiancaiamao @winkyao PTAL

[codecov]

Codecov Report

Merging #10152 into master will increase coverage by 0.0224%.
The diff coverage is 86.4077%.

@@               Coverage Diff                @@
##             master     #10152        +/-   ##
================================================
+ Coverage   78.2253%   78.2477%   +0.0224%     
================================================
  Files           413        413                
  Lines         87542      87623        +81     
================================================
+ Hits          68480      68563        +83     
+ Misses        13933      13922        -11     
- Partials       5129       5138         +9

[crazycs520]

@winkyao PTAL again.

[zimulala]

LGTM

[crazycs520]

/run-all-tests

[crazycs520]

/rebuild

[crazycs520]

/run-all-tests

[winkyao]

LGTM