planner: support explain and explain analyze statements for prepared queries

[qw4990]

Feature Request

Is your feature request related to a problem? Please describe:

Explain and explain analyze statements can print query plans, which is useful when optimizing queries and investigating issues, but prepared queries cannot support this feature:

mysql> explain execute stmt using @a,@b,@c;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 26 near "using @a,@b,@c"

Describe the feature you'd like:

Support explain and explain analyze statements for prepared queries, for example:

mysql> explain execute stmt using @a,@b,@c;
+--------------------------+---------+-----------+---------------+---------------------------------------------------------+
| id                       | estRows | task      | access object | operator info                                           |
+--------------------------+---------+-----------+---------------+---------------------------------------------------------+
| TableReader_7            | 0.04    | root      |               | data:Selection_6                                        |
| └─Selection_6            | 0.04    | cop[tikv] |               | ge(test.ta.a, "a"), ge(test.ta.b, 2), le(test.ta.b, 10) |
|   └─TableRangeScan_5     | 1.67    | cop[tikv] | table:ta      | range:["a",+inf], keep order:false, stats:pseudo        |
+--------------------------+---------+-----------+---------------+---------------------------------------------------------+
3 rows in set (0.63 sec)

Describe alternatives you've considered:

To get plans of prepared statements, now we can use explain for {ConnID} or search them from slow log file, but both these two methods are not convenient.

Teachability, Documentation, Adoption, Migration Strategy:

[qw4990]

Any options about this requirement? @zz-jason @eurekaka @winoros Since it is not complex and urgent, we can ask some contributors to implement this feature if it is accepted.

[LaPetiteSouris]

@qw4990 Question from a new comer here.

I tried to take a look into parser.y and it seems that we may have a few solutions

1. Either add ExecuteStmt into ExplainableStmt. This looks neats, but not sure if it is valid, as not all executable statements are explainable. Moreover, there are tons of shift/reduce conflicts to deal with in this case.

2. Do a customize:

   
   | ExplainSym "ANALYZE" ExecuteStmt
     {
         $$ = &ast.ExplainStmt{
             Stmt:    $3,
             Format:  "row",
             Analyze: true,
         }
     }

OR 

| ExplainSym ExecuteStmt { $$ = &ast.ExplainStmt{ Stmt: $2, Format: "row", } }

This approach works, however, it will duplicate a bit of code in `parse.y`

Another  question since I do not totally understand:

 `ExplainSym "ANALYZE" ExecuteStmt` does not generate any shift/reduce conflict, but `ExplainSym  ExecuteStmt` does. Do you have any suggestion ? I am quite blocked in this point, there must be something I miss. My best guess is that there is something to do with the `Identifier` argument in the definition of `ExecuteStmt`

ExecuteStmt: "EXECUTE" Identifier { $$ = &ast.ExecuteStmt{Name: $2} }

It seems that adding any valid keyword between `ExplainSym` and `ExecuteStmt` solves the shift/conflict problem (which is the case of `ExplainSym "ANALYZE" ExecuteStmt`. But I still do not understand why.

[LaPetiteSouris]

ExplainSym "ANALYZE" ExecuteStmt does not generate any shift/reduce conflict, but ExplainSym ExecuteStmt does. Do you have any suggestion ? I am quite blocked in this point, there must be something I miss. My best guess is that there is something to do with the Identifier argument in the definition of ExecuteStmt

ExecuteStmt:
  "EXECUTE" Identifier
  {
      $$ = &ast.ExecuteStmt{Name: $2}
  }

It seems that adding any valid keyword between ExplainSym and ExecuteStmt solves the shift/conflict problem (which is the case of ExplainSym "ANALYZE" ExecuteStmt. But I still do not understand why.

It seems that ExplainSym ExecuteStmt is ambiguous , as it is translated to EXPLAIN EXECUTE Identifier and Identifier can be anything from reserved to non reserved keyword, which is indeed ambiguous . I will try to address this issue in the PR

[valkyr13]

can i pick this up?

[LaPetiteSouris]

can i pick this up?

I don't have time to continue any further unfortunately, so feel free to grab. No objection for my side .