LLVM does not support SELECT without FROM currently

[hzxa21]

The LLVM engine does not support SELECT without FROM right now because it requires the projection plan to have a child plan. Here is the check:

peloton/src/codegen/operator/projection_translator.cpp

Line 26 in c30cb31

context.Prepare(*plan_.GetChild(0), pipeline);

If we use SELECT 1 FROM A where A is an empty table, the result will be empty. The reason might be that if the table is empty, the LLVM will not even generate codes for the projection logics.

[tcm-marcel]

Postgres also returns an empty result: http://sqlfiddle.com/#!17/5bfca/1/0

The codegen will generate code that evaluates the expression 1 once per row.
Empty table -> no rows to run the expression on -> empty result

[pmenon]

I agree with @tcm-marcel. The plan in that form (i.e., select 1 from A) has a scan over table A, and thus, the projection plan must have a child. Note that select 1 from A has different semantics that select 1: The former returns 1 for every row in A, while the latter returns only a single row 1.

[prashasthip]

peloton/src/codegen/query_compiler.cpp

Line 57 in c37483c

if (plan.GetChildren().empty()) return false;

This prevents codegening queries like:

select udf(parameter);

and it instead takes the normal execution path through Evaluate() in FunctionExpression.

[hzxa21]

@prashasthip You are right. I think the reason why we do this check is that codegen currently cannot execute ProjectionPlan with no children.