Lua表连接：按键关联的两表合并

-- 错误：双循环逐对比对，规模一大就拖
for _, m in ipairs(main) do
    for _, e in ipairs(extra) do
        if m.id == e.id then
            merge(m, e)
        end
    end
end

local function joinRows(main, extra, key)
    local idx = {}
    for _, e in ipairs(extra) do
        idx[e[key]] = e
    end
    local out = {}
    for _, m in ipairs(main) do
        out[#out + 1] = {
            id = m[key],
            name = m.name,
            ext = idx[m[key]] or {},
        }
    end
    return out
end