Recommendations for best SQL Performance updating and/or calculating stockonhand totals.

Question

Apologies for the length of this question.

I have a section of our database design which I am worried may begin to cause problems. It is not at that stage yet, but obviously don't want to wait until it is to resolve the issue. But before I start testing various scenarios, I would appreciate input from anyone who has experience with such a problem.

Situation is Stock Control and maintaining the StockOnHand value.

It would be possible to maintain a table hold the stock control figures which can be updated whenever a order is entered either manually or by using a database trigger. Alternatively you can get SQL to calculate the quantities by reading and summing the actual sales values.

The program is installed on several sites some of which are using MS-SQL 2005 and some 2008.

My problem is complicated because the same design needs to cope with several scenarios, such as :

1) Cash/Sale Point Of Sale Environment. Sale is entered and stock is reduced in one transaction. No amendments can be made to this transaction.

2) Order/Routing/Confirmation In this environment, the order is created and can be placed on hold, released, routed, amended, delivered, and invoiced. And at any stage until it is invoiced the order can be amended. (I mention this because any database triggers may be invoked lots of time and has to determine if changes should affect the stock on hand figures)

3) Different business have a different ideas of when their StockOnHand should be reduced. For example, some consider the stock as sold once they approve an order (as they have committed to sell the goods and hence it should not be sold to another person). Others do not consider the stock as sold until they have routed it and some others only when it has been delivered or collected.

4) There can be a large variance in number of transactions per product. For example, one system has four or five products which are sold several thousand times per month, so asking SQL to perform a sum on those transactions is reading ten of thousands of transactions per year, Whereas, on the same system, there are several thousand other products where sales would only less than a thousand transactions per year per product.

5) Historical information is important. For that reason, our system does not delete or archive transactions and has several years worth of transactions.

6) The system must have the ability to warn operators if they do not have the required stock when the order is entered ( which quite often is in real time, eg telephone order). Note that this only required for some products. (But I don't think it would be practical to sum the quantity across ten of thousands of transactions in real time).

7) Average Cost Price. Some products can be priced based on the average cost of the items in stock. The way this is implemented is that the Average Cost price is re-calculated for every goods in transaction, something like newAverageCostPrice = (((oldAverageCostPrice * oldStockOnHand) + newCostValue) / newStockOnHand) . This means the stock On Hand must be known for every goods in if the product is using AverageCost.

The way the system is currently implemented is two fold. We have a table which holds the StockOnHand for each product and location. Whenever a sale is updated, this table is updated via the business layer of our application (C#) This only provides the current stock on hand figures.

If you need to run a Stock Valuation for a particular date, this figure is calculated by performing a sum of the quantitys on the lines involved. This also requires a join between the sales line and the sale header tables as the quantity and product are stored in the line file and the date and status are only held in the header table.

However, there are downsides to this method, such as.

Running the stock valuation report is slow, (but not unacceptably slow), but I am not happy with it. (It works and monitoring the server does not show it overloading it, but it has the potential to cause problems and hence requires regular monitoring)

The logic of the code updating the StockOnHand table is complicated.

This table is being updated frequently. In a lot of cases this is un-necessary as the information does not need to be checked. For example, if 90% of your business is selling 4 or 5 products, you don't really need a computer to tell you are out of stock.

Database trigggers. I have never implemented complicated triggers before, so am wary of this. For example, as stated before we need configuration options to determine the conditions when the stock figures should be updated. This is currently read once and cached in our program. To do this inside a trigger would persumably mean reading this information for every trigger. Does this have a big impact on performance.

Also we may need a trigger on the sale header and the sale line. (This could mean that an amendment to the sale header would be forced to read the lines and update the stockonhand for the relevant products, and then later on the lines are saved and another database trigger would amend the stockonahand table again which may be in-efficient.

Another alternative would be to only update the StockOnHand table whenever the transaction is invoiced (which means no further amendments can be done) and to provide a function to calculate the stockonhand figure based on a union of this table and the un-invoiced transactions which affect stock.

Any advice would be greatly appreciated.

Answer 1

Could you create a view (or views) to respresent your stock on hand? This would take the responsibility for doing the calculations out of synchronous triggers which slow down your transactions. Using multiple views could satisfy the requirement "Different business have a different ideas of when their StockOnHand should be reduced." Assuming you can meet the stringent requirements, creating an indexed view could further improve your performance.

Answer 2

Just some ideas off the top of my head:

Instead of a trigger (and persistent SOH data), you could use a computed column (e.g. SOH per product per store). However, the performance impact of evaluating this would likely be abysmal unless there are >> more writes to your source tables than reads from your computed column. (The trade off is that is assuming the only reason you calculate the SOH is so that you can read it again. If you update the source data for the calc much more often than you actually need to read it, then the computed col might make sense - since it is JIT evaluation only when needed. This would be unusual though - reads are usually more frequent than writes in most Systems)

I'm guessing that the reason you are looking at triggers is because the source tables for the SOH figures are updated from a large number of procs / code in order to prevent oversight (as opposed to a calling a recalc SPROC from every applicable point where the source data has been modified?)

IMHO placing complicated in DB triggers is not advised, as this will adversely affect the performance of high volume inserts / updates, and triggers aren't great for maintainability.

Does the SOH calculation need to be real time? If not, you could implement a mechanism to queue requests for recalculation (e.g. by using a trigger to indicate that a product / location balance is dirty) and then run a recalculation service every few minutes for near real-time. Mission critical calculations (e.g. financial - like your #6) could still however detect that a SOH calc is dirty and then force a recalc before doing a transaction.

Re : 3 - Ouch. Would recommend that internally you agree on a consistent (and industry acceptable) set of terminology (Stock In Hand, Stock Committed, Stock in Transit, Shrinkage etc etc) and then try to convince your customers to conform to a standard. But that is in the ideal world of course!

Answer 3

First of I would strongly recommend you add "StockOnHand", "ReservedStock" and "SoldStock" to your table.

A cash sale would immediatly Subtract the sale from "StockOnHand" and add it to "SoldStock", for an order you would leave "StockOnHand" alone and merely add the sale to ReservedStock, when the stock is finally invoiced you substract the sale from StockOnHand and Reserved stock and add it to "SoldStock".

The business users can then choose whether StockOnHand is just that or StockOnHand - ReservedStock.

Using a maintaind StockOnHand figure will reduce your query times massively, versus the small risk that the figure can go out of kilter if you mess up your program logic.

If your customers are so lucky enough to experience update contention when maintaining the StockOnHand figure (i.e. are they likely to process more than five sales a second at peak times) then you can consisider the following scheme:-

Overnight calculate the StockOnHand figure by counting deliveries - sales or whatever. When a sale is confirmed insert a row to a "Todays Sales" table. When you need to query stock on hand total up todays sale and subtract it from the start of day figure.

You could also place a "Stock Check Threshold" on each product so if you start the day with 10,000 widgets you can set the CheckThreshold to 100 if someone is ordering less than 100 than dont bother checking the stock. If someone orders over 100 then check the stock and recalculate a new lower threshold.