Merging in PL/pgSQL and nested merges

A merge result stays loaded in memory (an expanded value) until it is stored, sent or cast to bytea, so chains of merges do not save and re-load the document at every step:

-- d is merged into in place; the document is saved once, at the UPDATE.
DECLARE d automerge; ch bytea;
BEGIN
    SELECT doc INTO d FROM docs WHERE id = $1 FOR UPDATE;
    FOREACH ch IN ARRAY change_sets LOOP
        d := merge(d, ch);
    END LOOP;
    UPDATE docs SET doc = d WHERE id = $1;
END;

On a 3 MB document, merging 20 change sets this way and storing the result takes 5.4 s, about two loads of the document in total; merge(merge(merge(doc, a), b), c), merge(...)::jsonb and merge_agg(...)::jsonb also skip the intermediate saves and loads. A single UPDATE .. SET doc = merge(doc, $1) gains nothing (the column arrives flat). A failed merge leaves the variable unchanged, also inside a BEGIN .. EXCEPTION block. Details in Expanded values.