It has previously been allowed for a subquery to use a tag within a
function (such as `count()`) when the tag is from a subquery and the
subquery itself references a field at some point to perform the join.
This functionality regressed in 1.6 because of a change in how
subqueries were executed that forgot to treat a tag the same as a string
field.
This fixes that regression and adds a test case to avoid hitting that
regression again.
This also fixes the cursor system to abandon iterators that will not
produce meaningful results since the variables are all unknown types.
This creates a weird behavior that existed in previous releases and we
are keeping here for backwards compatibility. If a subquery referenced a
field that didn't exist in the subquery, it will return nothing. But, if
there are two subqueries and one of them has the field exist and the
other doesn't, the second will return all null values.
The Cursor returned will be capable of scanning rows into a structure.
It replaces part of the function for why the Emitter existed. The
Emitter would both join the resulting rows and then transform the values
into a models.Row so it could be returned to the results.
In the future, we will be able to use the Cursor directly to write out
values which should be more memory efficient.
This change provides a clear separation between the query engine
mechanics and the query language so that the language can be parsed and
dealt with separate from the query engine itself.
Jonathan A. Sternberg
Jonathan A. Sternberg