Support table level comment

[shuiyisong]

What problem does the new feature solve?

Currently we only have column level comment. Adding table level comment can attach some information about the table, e.g: the table is auto-created by schemaless write from influxdb line protocol.

What does the feature do?

1. add option for comment on create table
2. display the comment in show create table

an example table option on creation could be

CREATE TABLE if not exists monitor (
  host STRING,
  ts TIMESTAMP(9) DEFAULT CURRENT_TIMESTAMP() TIME INDEX,
  cpu FLOAT64 DEFAULT 0,
  memory FLOAT64,
  PRIMARY KEY(host)) ENGINE=mito WITH(COMMENT='create by human');

Implementation challenges

tips:

1. see here for table option validation

[waynexia]

Reference: table comment parser in MySQL - https://dev.mysql.com/doc/refman/8.0/en/create-table.html

[imp2002]

I'm interested in this task, can you assign it to me?

[tisonkun]

If we use COMMENT="..." we can reuse the OptionMap code. I don't know if sqlparser-rs supports COMMENT "..." syntax now.

    /// Table options in `WITH`. All keys are lowercase.
    pub options: OptionMap,

[tisonkun]

It seems MySQL support COMMENT="..." also. We can thus reuse the options parse code and focus on conveying the comment.

I guess add the key "comment" in the allow list of pub fn validate_table_option(key: &str) -> bool should be a good start - FYI @imp2002

[waynexia]

Just recall that there is a relative field in TableInfo for storing table comments:

greptimedb/src/table/src/metadata.rs

Lines 655 to 657 in 777bc3b

	/// Comment of the table.
	#[builder(default, setter(into))]
	pub desc: Option<String>,

We can consider extracting the comment from table option and storing it here.