implement if [not] exists logic for DDL around views and indexes

[jycor]

This PR adds support for queries:

• create view if not exists ...
• create index if not exists ...
• alter table ... add index if not exists ...
• drop index if exists ...

fixes: dolthub/dolt#9293
companion pr: dolthub/vitess#417

[fulghum]

Looks good, but I think we should match MySQL 9.1's syntax for CREATE IF NOT EXISTS VIEW ... (even though I slightly prefer the current syntax) just for compatibility sake. Then we should decide if we want CREATE INDEX to be consistent with CREATE TABLE IF NOT EXISTS or CREATE IF NOT EXISTS VIEW.

[fulghum]

Sorry I didn't notice this while looking at the parser updates... it looks like we are slightly off of MySQL's syntax here... it seems like support for IF NOT EXISTS was added very recently, in MySQL 9.1. The docs show that that the form should be CREATE IF NOT EXISTS VIEW v AS SELECT 2;:
https://dev.mysql.com/doc/refman/9.1/en/create-view.html

I haven't tested this with MySQL 9.x, so it could be that the documentation is incorrect. I know that syntax is a little inconsistent with CREATE TABLE IF NOT EXISTS, too.

[fulghum]

I'm not sure MySQL actually supports this syntax, based on their docs:
https://dev.mysql.com/doc/refman/9.3/en/create-index.html

I tested with MySQL 8.4, but didn't try with MySQL 9.x.

That said, I think it's still useful and I'm not opposed to supporting it, although it will likely be hard for customers to discover if it isn't in the MySQL docs. We should just figure out if we want to be consistent with CREATE TABLE IF NOT EXISTS or CREATE IF NOT EXISTS VIEW, which unfortunately seems to be inconsistent in MySQL.

[jycor]

Yeah, I noticed this too.
MariaDB has been doing create view IF NOT EXISTS, which is consistent with all the other CREATE ... IF NOT EXISTS queries... it might be possible to do both?
So CREATE [IF NOT EXISTS] VIEW [IF NOT EXISTS] ...?

[jycor]

I downloaded MySQL 9.3 and tested it out; their docs are just messed up.

mysql> select version();
+-----------+
| version() |
+-----------+
| 9.3.0     |
+-----------+
1 row in set (0.0003 sec)

mysql> create if not exists view v as select 1;
ERROR: 1064: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'if not exists view v as select 1' at line 1

mysql> create view if not exists v as select 1;
Query OK, 0 rows affected (0.0044 sec)

mysql> select * from v;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.0009 sec)