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### What problem does this PR solve? <!--add issue link with summary if exists-->
https://github.com/pingcap/parser/issues/147https://github.com/pingcap/parser/pull/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 <!--REMOVE the items that are not applicable-->
Tests <!-- At least one of them must be included. -->
- Unit test
Code changes
Side effects
Related changes
- Need to cherry-pick to the release branch
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Originally, `fail` is added to simplify error handling, avoiding large changes on
the Txn() interface API. But it turns out that `fail` becomes the root of many bugs.
So this commit remove the `fail` and make an API refactory.
* change Txn() function signature to Txn(active bool)
* ActivePendingTxn() is not used any more because Txn() does the work
* change executor builder getStartTS() uint64 to getStartTS() (uint64, error)
`set @@autocommit = 0`
`select 1` // This statement should not make the session enter a transaction,
because it never need Txn().
Make txn lazy initialize
We import "github.com/pkg/errors" in the code, and actually put
the "github.com/pingcap/errors" there in the vendor path.
That's a dirty hack, and prevent the introduce of Go module.
Do not modify Plan of dataReaderBuilder directly, because it would
impact next batch of outer rows, as well as other concurrent inner
workers. Instead, build a local child builder to store the child plan.
