Danny Chen commented on CALCITE-963:

[~ScottReynolds] Sorry to bother, can you explain more about 
EnumerableRelImplementaor#stash ? I only see one direct invoke[1]

> This method takes an object stores it in DataContext and returns an 
> Expression that calls DataContext#get to pull out the object.

I didn't see a logic like what you described. It seems that 
EnumerableRelImplementaor#stash did some expression cache, but for constant, it 
always returns a constant expression directly.

> Hoist literals
> --------------
>                 Key: CALCITE-963
>                 URL: https://issues.apache.org/jira/browse/CALCITE-963
>             Project: Calcite
>          Issue Type: Bug
>            Reporter: Julian Hyde
>            Priority: Major
>              Labels: pull-request-available
>         Attachments: HoistedVariables.png
>          Time Spent: 0.5h
>  Remaining Estimate: 0h
> Convert literals into (internal) bind variables so that statements that 
> differ only in literal values can be executed using the same plan.
> In [mail 
> thread|http://mail-archives.apache.org/mod_mbox/calcite-dev/201511.mbox/%3c56437bf8.70...@gmail.com%3E]
>  Homer wrote:
> {quote}Imagine that it is common to run a large number of very similar 
> machine generated queries that just change the literals in the sql query.
> For example (the real queries would be much more complex):
> {code}Select * from emp where empno = 1;
> Select * from emp where empno = 2;
> etc.{code}
> The plan that is likely being generated for these kind of queries is going to 
> be very much the same each time, so to save some time, I would like to 
> recognize that the literals are all that have changed in a query and use the 
> previously optimized execution plan and just replace the literals.{quote}
> I think this could be done as a transform on the initial RelNode tree. It 
> would find literals (RexLiteral), replace them with bind variables 
> (RexDynamicParam) and write the value into a pool. The next statement would 
> go through the same process and the RelNode tree would be identical, but with 
> possibly different values for the bind variables.
> The bind variables are of course internal; not visible from JDBC. When the 
> statement is executed, the bind variables are implicitly bound.
> Statements would be held in a Guava cache.
> This would be enabled by a config parameter. Unfortunately I don't think we 
> could do this by default -- we'd lose optimization power because we would no 
> longer be able to do constant reduction.

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