Does Vespa support field projection for selected retrieval?

Vespa don't support this today but feature requests are welcome over at https://github.com/vespa-engine/vespa/issues

You can configure summary classes which is also more efficient then resolving this at query time. See https://docs.vespa.ai/en/document-summaries.html

document foo {
   field a ..
   field b ..
   field c ..
}

document-summary ab {
   field a .. { source: a}
   field b .. { source: b}
}