feat: implement DESCRIBE TABLE

[morigs]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Support DESC[RIBE] TABLE sql clause. Returns following info about table columns: name, type, nullable, default, semantic type.

Not sure about Display for Value implementation. Perhaps it should be more sql-ish

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

Resolves #534

[codecov]

Codecov Report

Merging #558 (232b816) into develop (8faa6b0) will increase coverage by 0.06%.
The diff coverage is 93.75%.

@@             Coverage Diff             @@
##           develop     #558      +/-   ##
===========================================
+ Coverage    86.38%   86.44%   +0.06%     
===========================================
  Files          398      399       +1     
  Lines        50042    50610     +568     
===========================================
+ Hits         43229    43752     +523     
- Misses        6813     6858      +45     

Flag	Coverage Δ
rust	86.44% <93.75%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/datanode/src/instance/sql.rs	86.58% <0.00%> (-4.45%)	⬇️
src/datanode/src/sql.rs	73.50% <0.00%> (-0.99%)	⬇️
src/frontend/src/instance.rs	73.31% <0.00%> (ø)
src/frontend/src/instance/distributed.rs	83.98% <0.00%> (+0.07%)	⬆️
src/query/src/datafusion/planner.rs	82.22% <ø> (ø)
src/query/src/error.rs	24.00% <0.00%> (+24.00%)	⬆️
src/sql/src/statements.rs	80.62% <ø> (ø)
src/sql/src/statements/statement.rs	48.27% <0.00%> (-5.58%)	⬇️
src/datatypes/src/schema/constraint.rs	97.38% <80.00%> (-0.47%)	⬇️
src/sql/src/parser.rs	71.69% <86.20%> (+1.78%)	⬆️
... and 17 more

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[waynexia]

Thanks @morigs ! This patch is very neat 👍

I've tried it in my local, it works as expect:

mysql> desc table t;
+--------+-----------+------+---------------------+---------------+
| Field  | Type      | Null | Default             | Semantic Type |
+--------+-----------+------+---------------------+---------------+
| host   | String    | NO   |                     | PRIMARY KEY   |
| ts     | Timestamp | NO   | current_timestamp() | TIME INDEX    |
| cpu    | Float64   | NO   | 0                   | VALUE         |
| memory | Float64   | YES  |                     | VALUE         |
+--------+-----------+------+---------------------+---------------+

(created by)

mysql> CREATE TABLE t (
    ->   host STRING,
    ->   ts TIMESTAMP DEFAULT current_timestamp,
    ->   cpu DOUBLE DEFAULT 0,
    ->   memory DOUBLE NULL,
    ->   TIME INDEX (ts),
    ->   PRIMARY KEY(host)) ENGINE=mito WITH(regions=1);
Query OK, 1 row affected (0.06 sec)

Not sure about Display for Value implementation. Perhaps it should be more sql-ish

Implement Display for Value looks good to me. In another patch I also use Display to draw SQL result table (here https://github.com/GreptimeTeam/greptimedb/blob/integration-test/tests/runner/src/util.rs#L3)

[morigs]

Thanks for the feedback. I'll fix this soon

[v0y4g3r]

Thanks for the feedback. I'll fix this soon

Currently I'm Ok with the catalog/schema name issue and marked these are resolved. Please fix other comments and we can merge this PR as soon as possible. Don't hesitate to reach us if you need any help!

[v0y4g3r]

LGTM