Add rule to eliminate LIMIT 0 and replace it with an EmptyRelation

[Dandandan]

Which issue does this PR close?

Closes #206

Rationale for this change

LIMIT 0 can be used to test a certain query / get the schema from a query or the value 0 can result from a constant folding. This rule replaces the limit 0 by an empty plan, saving time in the physical planner (no need to read metadata/statistics/partitions etc) and there is no need to produce a single batch.

What changes are included in this PR?

A new optimization pass to replace LIMIT 0 in the LogicalPlan with an EmptyRelation, with the same schema.

Are there any user-facing changes?

No breaking changes, should only be faster in some cases.

[returnString]

This looks great! I know lots of SQL client libs will issue select * from table limit 0-type queries to ascertain query result set descriptions, so definitely a worthwhile optimisation to avoid data source fetches even just in that instance 👍

[Dandandan]

This looks great! I know lots of SQL client libs will issue select * from table limit 0-type queries to ascertain query result set descriptions, so definitely a worthwhile optimisation to avoid data source fetches even just in that instance 👍

@returnString Cool, that was one of the use cases I was thinking about! Let me know when you experience some other queries that could benefit from a similar rule.

[codecov-commenter]

Codecov Report

Merging #213 (42ae033) into master (e86ad26) will increase coverage by 0.01%.
The diff coverage is 88.88%.

❗ Current head 42ae033 differs from pull request most recent head c165e42. Consider uploading reports for the commit c165e42 to get more accurate results

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
+ Coverage   76.30%   76.32%   +0.01%     
==========================================
  Files         134      135       +1     
  Lines       23170    23206      +36     
==========================================
+ Hits        17681    17713      +32     
- Misses       5489     5493       +4     

Impacted Files	Coverage Δ
datafusion/src/execution/context.rs	92.94% <ø> (ø)
datafusion/src/optimizer/eliminate_limit.rs	88.88% <88.88%> (ø)

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