Test and improve error messages for constraint expressions in opendap

[tdrwenski]

The open dap spec contains a section about constraint expressions, which includes queries like ?var&var>0. All types of expressions in the spec should work and be tested.

This does not currently work in TDS and is not tested either. This could be a regression, but as there are no tests or evidence that it used to work, I am labeling this as an enhancement.

[ethanrd]

The OPeNDAP DAP2 specification calls queries that look at data values "selection" constraint expressions. Array subsetting by index is called "projection" constraint expressions. I believe DAP2 only supports "selection" against the Sequence data type and "projection" on arrays.

DAP4 (specification) uses slightly different terminology for constraints. However, I believe arrays can still only be subset by index and that selection by value is not supported for arrays.

[tdrwenski]

Thanks for the clarification @ethanrd, I did not catch that those only applied for certain types. I am a bit confused about simple data types like int, float, etc. In the Table 5 of the DAP2 spec it seems like the "selection" constraints should apply in that case, but when I tested this in TDS I get a 500 error.

[tdrwenski]

Okay, I may have been testing on an array of floats, which could be type Array? I am confused why you would have to use the selection operators on a float that is not an array. I guess it may only make sense for sequences.

(@DennisHeimbigner in case this is relevant for you too)

[ethanrd]

A Sequence in OPeNDAP is a structure that may have many instances. So this dataset

Dataset {
    Int32 u;
    Int32 v;
    Sequence {
        Int32 x;
        Int32 y;
    } Point;
} vol_1_ce_1;

might have 100 Points (the size isn't known till you request and receive it). A request for Point.x>5 would return all the Points that have x values greater than 5, however many that might be.

[ethanrd]

Oops! Looks like I explained more than needed. Sorry! Yes, as far as I know those types of queries are mainly useful for Sequences.

[tdrwenski]

Thanks Ethan! That is clear now! The current behavior may be correct then (I have not thoroughly tested it though).

Maybe then I will revise this issue to:

• Improve the error message given when a selection operator is used on the wrong type. It should not be a 500 error like we are getting now.
• Ensure there are tests for all types of constraint expressions and data types

[DennisHeimbigner]

Good catch, Ethan. I assumed the query was against a sequence. Ethan is correct that only
sequences should support relational expressions.