Technique #3: eliminating recursive embedded SQL

It is actually quite common to find embedded SQL in a loop, especially Select and Insert statements. As explained previously, server calls that are recursive in nature are quite dangerous, potentially generating tremendous number of server calls. If your application requires loops or recursive functions, it would be best to replace any code resulting in server calls with code that does not.

For this technique, we will assume we have Select and Insert SQL statements in a loop. The general idea is to first create a DataWindow/DataStore using the SQL. Then replace the SQL statements contained in the loop with PowerScript modifying the DataWindow/DataStore, which does not result in server calls. If the SQL statement contained in the loop is an Insert statement, we would want to replace that with PowerScript that would insert data into the DataWindow/DataStore. Once all the data has been inserted, then in one shot we would update the DataWindow/DataStore to the database (outside the loop), resulting in only one server call. If the SQL statement contained in the loop is a Select statement, we would retrieve data into a DataWindow/DataStore before executing the loop, and then write PowerScript in the loop to select the desired data from the DataWindow/DataStore.

The following is a code example that increases the price of a specific order by 20%, where embedded SQL is used to update the change row-by-row (hence the loop), and then save those changes to the database:

long ll_id

declare order_detail cursor for 
select id from order_detail where orderid = :arg_orderid;
open order_detail;
fetch order_detail into :ll_id;

do while sqlca.sqlcode = 0
	update order_detail set price = price*1.2
	where orderid = :arg_orderid and id = :ll_id; 

	if sqlca.sqlcode < 0 then
		rollback;
		return
	end if
	
	fetch order_detail into :ll_id;
loop
close order_detail;
commit;

Now we will replace the embedded SQL with a DataWindow. Specifically, we will cache the data in a DataWindow and update the database with a single DataWindow Update, resulting in just once server call:

long ll_rows, i

ll_rows = dw_1.retrieve(arg_orderid)
for i = 1 to ll_rows
    dw_1.SetItem(i, "price", dw_1.GetItemDecimal(i, "price")*1.2)
next

if dw_1.update() = 1 then
commit;
else
rollback;
end if

With this technique we have just eliminated server calls from inside the loop, reduced the number of server calls to just one, and created a data caching mechanism at the client-side that can be used to feed data to other controls of the PowerBuilder client.