I'd like to add some things to dynamic_search(): limitby - improves performance on large tables, and orderby so that i can sort. Also, i added id and class to the input so i get the date-picker widget automagically for dates. There were a couple of changes i made since i'm using it on Google App Engine as well.

def dynamic_search(table):
    """
    Build the input for for selection criteria, and runs a query
    based on the current criteria
    """
    tbl = TABLE()
    selected = []
    ops = ['equals','not equal','greater than','less than',
           'starts with','ends with','contains']
    if request.env.web2py_runtime_gae:
        ops = ['equals','not equal','greater than','less than']

    query = table.id > 0    
    for field in table.fields:
        chkval = request.vars.get('chk'+field,None)
        txtval = request.vars.get('txt'+field,None)
        opval = request.vars.get('op'+field,None)
        row = TR(TD(INPUT(_type="checkbox",_name="chk"+field,
                          value=chkval=='on')),
                 TD(field),TD(SELECT(ops,_name="op"+field,
                                     value=opval)),
                 TD(INPUT(_type="text",_name="txt"+field,_id="txt"+field,
                          _value=txtval, _class=table[field].type)))
        tbl.append(row)
        if chkval:
            if txtval:
                query &= build_query(table[field], 
                                opval,txtval)
            selected.append(table[field])           

    #limit the results
    row = TR(TD(), TD('Limit to'), TD(),
             TD(INPUT(_type="text",_name="limitby", _id="limitby",
                      _value=request.vars.get('limitby', "100"),
                      _class="string")))
    tbl.append(row)

    #order the requelts
    opts = table.fields[:]
    opts.remove("id")
    opts = [""] + opts
    row = TR(TD(), TD('Order by'), TD(),
             TD(SELECT(opts,_name="orderby",
                       value=request.vars.get('orderby', None))))
    tbl.append(row)


    form = FORM(tbl,INPUT(_type="submit"))


    #for GAE the orderby must already be a table field, not a string, so convert
    orderby=None
    if request.vars.orderby:
        field = request.vars.orderby
        orderby=db[table][field]

    results=db(query).select(limitby=(0,int(request.vars.get('limitby','100'))), 
                                        orderby=orderby, *selected)

    return form, results