[jira] [Created] (IMPALA-7914) Introduce AST base class/interface for statement-like nodes

["Paul Rogers (JIRA)" <ji...@apache.org>]

Paul Rogers created IMPALA-7914:
-----------------------------------

             Summary: Introduce AST base class/interface for statement-like nodes
                 Key: IMPALA-7914
                 URL: https://issues.apache.org/jira/browse/IMPALA-7914
             Project: IMPALA
          Issue Type: Improvement
          Components: Frontend
    Affects Versions: Impala 3.0
            Reporter: Paul Rogers
            Assignee: Paul Rogers


The front-end is based on an abstract syntax tree (AST). The parser creates the AST, the analyzer decorates it with semantic information, and the planner creates a plan for it.

At present, the class hierarchy looks like this:

{noformat}
ParseNode
|-- Expr
|   |-- <all expression nodes>
|-- FromClause
|-- <other statment fragments>
|-- StatementBase
|   |-- SelectStmt
|   |-- <other statements>
{noformat}

This is a nuisance because the only common base class for all statement-like nodes is the {{ParseNode}}, which is also the common base class or expressions. However, expressions and statement-like nodes behave differently during analysis, SQL generation, and so on.

We propose to refactor the tree to introduce a new {{StmtNode}} interface or class that defines the statement-like semantics, leaving {{Expr} to define the expression-like semantics. The methods then move out of {{ParseNode}}.

This change all allow revising the analysis step as follows:

* Analysis of statement-like nodes is done "in place"
* Analysis of expression nodes may result in replacing one node with a rewritten version

Similarly, when generating SQL:

* Statements provide the option of generating before- or after-rewrite SQL.
* Expressions can only generate SQL for what they are; they have no concept of before- or after- rewrites.

Specifically:

{noformat}
ParseNode
|-- Expr
|   |-- <all expression nodes>
|-- StmtNode
|   |-- FromClause
|   |-- <other statment fragments>
|   |-- StatementBase
|   |   |-- SelectStmt
|   |   |-- <other statements>
{noformat}

It may be useful to introduce a {{ClauseNode}}, but we'll see if that is actually helpful as we do the refactoring:

{noformat}
|-- StmtNode
|   |-- ClauseNode
|   |   |-- FromClause
|   |   |-- <other statment fragments>
|   |-- StatementBase
|   |   |-- SelectStmt
|   |   |-- <other statements>
{noformat}



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