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One of the heuristics for test files looks for source files of the form `base.ext`, then looks for sibling test files of the form `base.test.ext` or `base.spec.ext`. On large databases, the result join order computed all source files, the containers of those files, then all other files within those containers, before computing the test file names and filtering using those names. The product of all files with all other files in the same containers is of the same order of magnitude as the product of the `files` table with itself, which on large DBs like Node can be 12M+ tuples. As a performance optimisation, factor out a helper predicate that computes the likely test file names for each source file, so these can be determined with a single join against the files table. This results in much better join orders, such as computing the set of files and their containers, then the test file names, then the sibling files with those names. This loses some flexibility because the set of 'test' extension names is hardcoded in the library rather than provided by the caller predicate. The original predicate remains to avoid breaking other callers, but could eventually be deprecated.