Implement DESCRIBE TABLE clause

[waynexia]

What problem does the new feature solve?

Describe the structure of a table, as well as the default values.

What does the feature do?

Support syntax like

DESC[RIBE] TABLE <name>;

It returns a table like this:

Name               Null             Type                Default        Semantic Type

ID                    Not Null        INT                    0           PRIMARY KEY
Timestamp                            TIMESTAMP                          TIME INDEX

Implementation challenges

• Add these syntaxes to parser
• Retrieve TableInfo from corresponding table
• Extract infos from TableInfo and output them.

[morigs]

Hi, I was experimenting while exploring the source code and now I have almost complete implementation.
This missing piece of puzzle is how to get semantic type from ColumnSchema. If I'm not mistaken, it should be TIMESTAMP when is_time_index is true. But what about TAG? Should I treat fields from TableMeta.primary_key_indices as TAGs and the rest as FIELDs?

[waynexia]

Should I treat fields from TableMeta.primary_key_indices as TAGs and the rest as FIELDs?

That's right! Sorry that the document isn't clear. Primary key and tag are the same thing here.

And since the CREATE TABLE clause uses PRIMARY KEY, I think I'd better to change the PR description to use this word as well. So the three possible value of "semantic type" here are TIME INDEX(not timestamp), PRIMARY KEY and VALUE (not field).

[morigs]

👍🏼 Thx, I'll try to implement this soon. Could you pls assign this to me?