The PostgreSQL source code can be compiled with coverage testing
    instrumentation, so that it becomes possible to examine which
    parts of the code are covered by the regression tests or any other
    test suite that is run with the code.  This is currently supported
    when compiling with GCC and requires the gcov
    and lcov programs.
   
A typical workflow would look like this:
./configure --enable-coverage ... OTHER OPTIONS ... make make check # or other test suite make coverage-html
    Then point your HTML browser
    to coverage/index.html.
    The make commands also work in subdirectories.
   
To reset the execution counts between test runs, run:
make coverage-clean