planner,executor: support show create database if not exists syntax

[lonng]

What problem does this PR solve?

pingcap/parser#147
pingcap/parser#148

MySQL compatibility: show create database if not exists syntax

See: https://dev.mysql.com/doc/refman/5.7/en/show-create-database.html

MySQL

MariaDB [(none)]> show create database b2;
+----------+----------------------------------------------------------------+
| Database | Create Database                                                |
+----------+----------------------------------------------------------------+
| b2       | CREATE DATABASE `b2` /*!40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+----------------------------------------------------------------+
1 row in set (0.002 sec)

MariaDB [(none)]> show create database if not exists b2;
+----------+-----------------------------------------------------------------------------------------+
| Database | Create Database                                                                         |
+----------+-----------------------------------------------------------------------------------------+
| b2       | CREATE DATABASE /*!32312 IF NOT EXISTS*/ `b2` /*!40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+-----------------------------------------------------------------------------------------+
1 row in set (0.000 sec)

TiDB

MySQL [b1]> show create batabase b2;
ERROR 1105 (HY000): line 1 column 20 near " b2" (total length 23)
MySQL [b1]> show create database b2;
+----------+-----------------------------------------------------------------+
| Database | Create Database                                                 |
+----------+-----------------------------------------------------------------+
| b2       | CREATE DATABASE `b2` /* !40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+-----------------------------------------------------------------+
1 row in set (0.000 sec)

MySQL [b1]> show create database if not exists b2;
ERROR 1105 (HY000): line 1 column 23 near " not exists b2" (total length 37)

What is changed and how it works?

Insert /*!32312 IF NOT EXISTS*/ before shema name when statement contains if not exists

Check List

Tests

• Unit test

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

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This change is 

[lonng]

PTAL @winkyao @tiancaiamao @eurekaka @kennytm

[tiancaiamao]

Please update go.mod

[lonng]

/run-all-tests

[winkyao]

LGTM

[eurekaka]

LGTM

[eurekaka]

/run-unit-test

[zyguan]

/run-unit-test

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@cef6e75). Click here to learn what that means.
The diff coverage is 28.57%.

@@            Coverage Diff            @@
##             master    #8926   +/-   ##
=========================================
  Coverage          ?   67.54%           
=========================================
  Files             ?      363           
  Lines             ?    74989           
  Branches          ?        0           
=========================================
  Hits              ?    50649           
  Misses            ?    19880           
  Partials          ?     4460

Impacted Files	Coverage Δ
planner/core/common_plans.go	60.98% <ø> (ø)
executor/show.go	40.85% <0%> (ø)
planner/core/planbuilder.go	48.68% <100%> (ø)
executor/builder.go	82.77% <100%> (ø)

---

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[lonng]

/run-all-tests

[zimulala]

/run-unit-test