Lookup Transformation in SSIS
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THE LOOKUP TRANSFORMATION
The Lookup Transformation in SSIS enables you to perform the similar relational inner and outer hash-joins. The main difference is that the operations occur outside the realm of the database engine and in the SSIS Data Flow. Typically, you would use this component within the context of an integration process, such as the ETL layer that populates a data warehouse from source systems. For example, you may want to populate a table in a destination system by joining data from two separate source systems on different database platforms.
The component can join only two data sets at a time, so in order to join three or more data sets, you would need to chain multiple Lookup Transformations together, using an output from one Lookup Transformation as an input for another. Compare this to relational join semantics, whereby in a similar fashion you join two tables at a time and compose multiple such operations to join three or more tables.
The transformation is written to behave in a synchronous manner, meaning it does not block the pipeline while it is doing its work. While new rows are entering the Lookup Transformation, rows that have already been processed are leaving through one of four outputs. However, there is a catch here: in certain caching modes (discussed later in this Joining Data Topic) the component will initially block the package’s execution for a period of time while it loads its internal caches with the Lookup data.
The component provides several modes of operation that enable you to compare performance and resource usage. In full-cache mode, one of the tables you are joining is loaded in its entirety into memory, and then the rows from the other table are flowed through the pipeline one buffer at a time, and the selected join operation is performed. With no up-front caching, each incoming row in the pipeline is compared one at a time to a specified relational table. Between these two options is a third that combines their behavior. Each of these modes is explored later in this Post (see the “Full- Cache Mode,” “No-Cache Mode,” and “Partial-Cache Mode” sections).
Of course, some rows will join successfully, and some rows will not be joined. For example, consider a customer who has made no purchases. His or her identifier in the Customer table would have no matches in the sales table. SSIS supports this scenario by having multiple outputs on the Lookup Transformation. In the simplest (default/legacy) configuration, you would have one output for matched rows and a separate output for nonmatched and error rows. This functionality enables you to build robust (error-tolerant) processes that, for instance, might direct nonmatched rows to a staging area for further review. Or the errors can be ignored, and a Derived Column Transformation can be used to check for null values. A conditional statement can then be used to add default data in the Derived Column. A more detailed example is given later in this Post.
The Cache Connection Manager (CCM) is a separate component that is essential when creating advanced Lookup operations. The CCM enables you to populate the Lookup cache from an arbitrary source; for instance, you can load the cache from a relational query, an Excel file, a text file, or a Web service. You can also use the CCM to persist the Lookup cache across iterations of a looping operation. You can still use the Lookup Transformation without explicitly using the CCM, but you would then lose the resource and performance gains in doing so. CCM is described in more detail later in this Joining Data Topic.
USING THE LOOKUP TRANSFORMATION
The Lookup Transformation solves join differently than the Merge Join Transformation. The Lookup Transformation typically caches one of the data sets in memory, and then compares each row arriving from the other data set in its input pipeline against the cache. The caching mechanism is highly configurable, providing a variety of different options in order to balance the performance and resource utilization of the process.
In full-cache mode, the Lookup Transformation stores all the rows resulting from a specified query in memory. The benefit of this mode is that Lookups against the in-memory cache are very fast — often an order of magnitude or more, relative to a no-cache mode Lookup. Full-cache mode is the default because in most scenarios it has the best performance of all of the techniques discussed in the Topic.
Continuing with the example package you built in the previous section (“Using the Merge Join Transformation”), you will in this section extend the existing package in order to join the other required tables. You already have the related values from the order header and order detail tables, but you still need to map the natural keys from the Product and Customer tables. You could use Merge Join Transformations again, but this example demonstrates how the Lookup Transformation can be of use here:
- Open the package you created in the previous step. Remove the Union All Transformation. Drop a Lookup Transformation on the surface, name it LKP Customer, and connect the output of the Merge Join Transformation to it. Open the editor of the Lookup Transformation.
- Select Full-Cache Mode, specifying an OLE DB Connection Manager. There is also an option to specify a Cache Connection Manager (CCM), but you won’t use this just yet — later in this Joining Data Topic you will learn how to use the CCM. (After you have learned about the CCM, you can return to this
exercise and try to use it here instead of the OLE DB Connection Manager.)
- Click the Connection tab and select the AdventureWorks connection, and then use the following SQL query:select CustomerID, AccountNumber
- Preview the results to ensure that everything is set up OK, then click the Columns tab. Drag the CustomerID column from the left-hand table over to the CustomerID column on the right; this creates a linkage between these two columns, which tells the component that this column is used to
perform the join. Click the checkbox next to the AccountNumber column on the right, which tells the component that you want to retrieve the AccountNumber values from the Customer table for each row it compares. Note that it is not necessary to retrieve the CustomerID values from the
right-hand side because you already have them from the input columns. The editor should now look like shown screenshot below.
- Click OK on the dialog, hook up a “trash” Union All Transformation . Create a Data Viewer on the match output path of the Lookup Transformation and execute the package (you could also attach a Data Viewer on the no-match output and error output if needed). You should see results similar to the below screen shot. Notice you have all the columns from the order and details data, as well as the selected column from the Customer table.
Because the Customer table is so small and the package runs so fast, you may not have noticed what happened here. As part of the pre-execution phase of the component, the Lookup Transformation fetched all the rows from the Customer table using the query specified (because the Lookup was configured to execute in full-cache mode). In this case there are only 20,000 or so rows, so this happens very quickly. Imagine that there were many more rows, perhaps two million. In this case you would likely experience a delay between executing the package and seeing any data actually traveling down the second pipeline.below the screen shot shows a decision tree that demonstrates how the Lookup Transformation in full-cache mode operates at runtime. Note that the Lookup Transformation can be configured to send found and not-found rows to the same output, but the illustration assumes they are going to different outputs. In either case, the basic algorithm is the same.Check the Execution Results tab on the SSIS design surface (see below screenshot) and see how long it took for the data to be loaded into the in-memory cache. In larger data sets this number will be much larger and could even take longer than the execution of the primary functionality!
Note:If during development and testing you want to emulate a longrunning query, use the T-SQL waitfor statement in the query in the following manner.
waitfor delay ’00:00:059′; –Wait 5 seconds before returning any
select CustomerID, AccountNumber
After fetching all the rows from the specified source, the Lookup Transformation caches them in memory in a special hash structure. The package then continues execution; as each input row enters the Lookup Transformation, the specified key values are compared to the in-memory hash values, and, if a match is found, the specified return values are added to the output stream.
If the reference table (the Customer table in this case) is too large to cache all at once in the system’s memory, you can choose to cache nothing or you can choose to cache only some of the data. This section explores the first option: no-cache mode.
In no-cache mode, the Lookup Transformation is configured almost exactly the same as in full-cache mode, but at execution time the reference table is not loaded into the hash structure. Instead, as each input row flows through the Lookup Transformation, the component sends a request to the reference table in the database server to ask for a match. As you would expect, this can have a high performance overhead on the system, so use this mode with care.
Depending on the size of the reference data, this mode is usually the slowest, though it scales to the largest number of reference rows. It is also useful for systems in which the reference data is highly volatile, such that any form of caching would render the results stale and erroneous.
Below the Screen shot illustrates the decision tree that the component uses during runtime. As before, the diagram assumes that separate outputs are configured for found and not-found rows, though the algorithm would be the same if all rows were sent to a single output.
Here are the steps to build a package that uses no-cache mode:
- Rather than build a brand-new package to try out no-cache mode, use the package you built in the previous section (“Full-Cache Mode”). Open the editor for the Lookup Transformation and on the first tab (General), choose the No-Cache option. This mode also enables you to customize (optimize) the query that SSIS will submit to the relational engine. To do this, click the Advanced tab and check the Modify the SQL Statement checkbox. In this case, the auto-generated statement is close enough to optimal, so you don’t need to touch it. (If you have any problems
reconfiguring the Lookup Transformation, then delete the component, drop a new Lookup on the design surface, and reconnect and configure it from scratch.)
- Execute the package. It should take slightly longer to execute than before, but the results should be the same.
The trade-off you make between the caching modes is one of performance versus resource utilization. Full-cache mode can potentially use a lot of memory to hold the reference rows in memory, but it is usually the fastest because Lookup operations do not require a trip to the database. No-cache mode, on the other hand, requires next to no memory, but it’s slower because it requires a database call for every Lookup. This is not a bad thing; if your reference table is volatile (i.e., the data changes often), you may want to use no-cache mode to ensure that you always have the latest version of each row.
Partial-cache mode gives you a middle ground between the no-cache and full-cache options. In this mode, the component caches only the most recently used data within the memory boundaries specified under the Advanced tab in the Lookup Transform. As soon as the cache grows too big, the least-used cache data is thrown away.
When the package starts, much like in no-cache mode, no data is preloaded into the Lookup cache. As each input row enters the component, it uses the specified key(s) to attempt to find a matching record in the reference table using the specified query. If a match is found, then both the key and the Lookup values are added to the local cache on a just-in-time basis. If that same key enters the Lookup Transformation again, it can retrieve the matching value from the local cache instead of the reference table, thereby saving the expense and time incurred of requerying the database.
In the example scenario, for instance, suppose the input stream contains a CustomerID of 123. The first time the component sees this value, it goes to the database and tries to find it using the specified query. If it finds the value, it retrieves the AccountNumber and then adds the CustomerID/AccountNumber combination to its local cache. If CustomerCD 123 comes through again later, the component will retrieve the AccountNumber directly from the local cache instead of going to the database.
If, however, the key is not found in the local cache, the component will check the database to see if it exists there. Note that the key may not be in the local cache for several reasons: maybe it is the first time it was seen, maybe it was previously in the local cache but was evicted because of memory pressure, or finally, it could have been seen before but was also not found in the database.
For example, if CustomerID 456 enters the component, it will check the local cache for the value. Assuming it is not found, it will then check the database. If it finds it in the database, it will add 456 to its local cache. The next time CustomerID 456 enters the component, it can retrieve the value directly from its local cache without going to the database. However, it could also be the case that memory pressure caused this key/value to be dropped from the local cache, in which case the component will incur another database call.
If CustomerID 789 is not found in the local cache, and it is not subsequently found in the reference table, the component will treat the row as a nonmatch, and will send it down the output you have chosen for nonmatched rows (typically the no-match or error output). Every time that CustomerID 789 enters the component, it will go through these same set of operations. If you have a high degree of expected misses in your Lookup scenario, this latter behavior — though proper and expected — can be a cause of long execution times because database calls are expensive relative to a local cache check.
To avoid these repeated database calls while still getting the benefit of partialcache mode, you can use another feature of the Lookup Transformation: the miss cache. Using the partial-cache and miss-cache options together, you can realize further performance gains. You can specify that the component remembers values that it did not previously find in the reference table, thereby avoiding the expense of looking for them again. This feature goes a long way toward solving the performance issues discussed in the previous paragraph, because ideally every key is looked for once — and only once — in the reference table.
To configure this mode, follow these steps (refer to below screenshot):
- Open the Lookup editor, and in the General tab select the Partial Cache option. In the Advanced tab, specify the upper memory boundaries for the cache and edit the SQL statement as necessary. Note that both 32-bit and 64-bit boundaries are available because the package may be built and
tested on a 32-bit platform but deployed to a 64-bit platform, which has more memory. Providing both options makes it simple to configure the component’s behavior on both platforms.
- If you want to use the miss-cache feature, configure what percentage of the total cache memory you want to use for this secondary cache (say,25%).
The decision tree shown in Figure 7-22 demonstrates how the Lookup Transformation operates at runtime when using the partial-cache and misscache options. Note that some of the steps are conceptual; in reality, they are implemented using a more optimal design. As per the decision trees shown for the other modes, this illustration assumes separate outputs are used for the found and not-found rows.
At this point, your Lookup Transformation is working, and you have learned different ways to optimize its performance using fewer or more resources. In this section, you’ll learn how to utilize some of the other features in the component, such as the different outputs that are available.
Using the same package you built in the previous sections, follow these steps:
- Reset the Lookup Transformation so that it works in full-cache mode. It so happens that, in this example, the data is clean and thus every row finds a match, but you can emulate rows not being found by playing quick and dirty with the Lookup query string. This is a useful trick to use at
design time in order to test the robustness and behavior of your Lookup Transformations. Change the query statement in the Lookup Transformation as follows:select CustomerID, AccountNumber
where CustomerID % 7 <> 0; –Remove 1/7 of the rows
- Run the package again. This time, it should fail to execute fully because the cache contains one-seventh fewer rows than before, so some of the incoming keys will not find a match, as shown in Below Screenshot. Because the default error behavior of the component is to fail on any nonmatch or error condition such as truncation, the Lookup halts as expected.
Try some of the other output options. Open the Lookup editor and on the dropdown listbox in the General tab, choose how you want the Lookup Transformation to behave when it does not manage to find a matching join entry:
->Fail Component should already be selected. This is the default behavior, which causes the component to raise an exception and halt execution if a nonmatching row is found or a row causes ->an error such as data truncation. Ignore Failure sends any nonmatched rows and rows that cause errors down the same output as the matched rows, but the Lookup values (in this case AccountNumber) will be set to null. If you add a Data Viewer to the flow, you should be able to see this; several of the AccountNumbers will have null values.
->Redirect Rows to Error Output is provided for backward compatibility with SQL Server 2005. It causes the component to send both nonmatched and error-causing rows down the same error (red) output.
->Redirect Rows to No Match Output causes errors to flow down the error (red) output, and no-match rows to flow down the no-match output.
- Choose Ignore Failure and execute the package. The results should look like the below screen shot. You can see that the number of incoming rows on the Lookup Transformation matches the number of rows coming out of its match output, even though one-seventh of the rows were not actually matched. This is because the rows failed to find a match, but because you configured the Ignore Failure option, the component did not stop execution.
- Open the Lookup Transformation and this time select “Redirect rows to error output.” In order to make this option work, you need a second trash destination on the error output of the Lookup Transformation, as shown in below screen shot. When you execute the package using this mode, the found rows will be sent down the match output, and unlike the previous modes, not-found rows will not be ignored or cause the component to fail but will instead be sent down the error output.
- Finally, test the “Redirect rows to no match output” mode. You will need a total of three trash destinations for this to work, as shown in below screenshot.
In all cases, add Data Viewers to each output, execute the package, and examine the results. The outputs should not contain any errors such as truncations, though there should be many nonmatched rows. So how exactly are these outputs useful? What can you do with them to make your packages more robust? In most cases, the errors or nonmatched rows can be piped off to a different area of the package where the values can be logged or fixed as per the business requirements. For example, one common solution is for all missing rows to be tagged with an Unknown member value. In this scenario, all nonmatched rows might have their AccountNumber set to 0000. These fixed values are then joined back into the main Data Flow and from there treated the same as the rows that did find a match. Use the following steps to configure the package to do this:
- Open the Lookup editor. On the General tab, choose the “Redirect rows to no match output” option. Click the Error Output tab (see in below screenshot) and configure the AccountNumber column to have the value Fail Component under the Truncation column. This combination of settings means that you want a no-match output, but you don’t want an error output; instead you want the component to fail on any errors. In a real-world scenario, you may want to have an error output that you can use to log values to an error table, but this example keeps it simple.
- At this point, you could drop a Derived Column Transformation on the design surface and connect the no-match output to it. Then you would add the AccountNumber column in the derived column, and use a Union All to bring the data back together. This approach works, but the partially blocking Union All slows down performance.
However, there is a better way to design the Data Flow. Set the Lookup to Ignore Errors. Drop a Derived Column on the Data Flow. Connect the match output to the derived column. Open the Derived Column editor and replace the AccountNumber column with the following expression.
The Derived Column Transformation dialog editor should now look something like below the screen shot.
Close the Derived Column editor, and drop a Union All Transformation on the surface. Connect the default output from the Derived Column to the Union All Transformation and then execute the package, as usual utilizing a Data Viewer on the final output. The package and results should look something like Below the screen shot.
The output should show AccountNumbers for most of the values, with 0000 shown for those keys that are not present in the reference query (in this case because you artificially removed them).
If you need to build a package whose required reference table is not known at design time, this feature will be useful for you. Instead of using a static query in the Lookup Transformation, you can use an expression, which can dynamically construct the query string, or it could load the query string using the parameters feature.
below the screenshot shows an example of using an expression within a Lookup Transformation. Expressions on Data Flow Components can be accessed from the property page of the Data Flow Task itself.
Cascaded Lookup Operations
Sometimes the requirements of a real-world Data Flow may require several Lookup Transformations to get the job done. By using multiple Lookup Transformations, you can sometimes achieve a higher degree of performance without incurring the associated memory costs and processing times of using a single Lookup.
Imagine you have a large list of products that ideally you would like to load into one Lookup. You consider using full-cache mode; however, because of the sheer number of rows, either you run out of memory when trying to load the cache or the cache-loading phase takes so long that it becomes impractical (for instance, the package takes 15 minutes to execute, but 6 minutes of that time is spent just loading the Lookup cache). Therefore, you consider nocache mode, but the expense of all those database calls makes the solution too slow. Finally, you consider partial-cache mode, but again the expense of the initial database calls (before the internal cache is populated with enough data to be useful) is too high.
The solution to this problem is based on a critical assumption that there is a subset of reference rows (in this case product rows) that are statistically likely to be found in most, if not all, data loads. For instance, if the business is a consumer goods chain, then it’s likely that a high proportion of sales transactions are from people who buy milk. Similarly, there will be many transactions for sales of bread, cheese, beer, and baby diapers. On the contrary, there will be a relatively low number of sales for expensive wines. Some of these trends may be seasonal — more suntan lotion sold in summer, and more heaters sold in winter. This same assumption applies to other dimensions besides products — for instance, a company specializing in direct sales may know historically which customers (or customer segments or loyalty members) have responded to specific campaigns. A bank might know which accounts (or account types) have the most activity at specific times of the month.
This statistical property does not hold true for all data sets, but if it does, you may derive great benefit from this pattern. If it doesn’t, you may still find this section useful as you consider the different ways of approaching a problem and solving it with SSIS.
So how do you use this statistical approach to build your solution? Using the consumer goods example, if it is the middle of winter and you know you are not going to be selling much suntan lotion, then why load the suntan products in the Lookup Transformation? Rather, load just the high-frequency items like milk, bread, and cheese. Because you know you will see those items often, you want to put them in a Lookup Transformation configured in fullcache mode. If your Product table has, say, 1 million items, then you could load the top 20% of them (in terms of frequency/popularity) into this first Lookup. That way, you don’t spend too much time loading the cache (because it is only 200,000 rows and not 1,000,000); by the same reasoning, you don’t use as much memory.
Of course, in any statistical approach there will always be outliers — for instance, in the previous example suntan lotion will still be sold in winter to people going on holiday to sunnier places. Therefore, if any Lookups fail on the first full-cache Lookup, you need a second Lookup to pick up the strays. The second Lookup would be configured in partial-cache mode (as detailed earlier in this Post), which means it would make database calls in the event that the item was not found in its dynamically growing internal cache. The first Lookup’s not-found output would be connected to the second Lookup’s input, and both of the Lookups would have their found outputs combined using a Union All Transformation in order to send all the matches downstream. Then a third Lookup is used in no-cache mode to look up any remaining rows not found already. This final Lookup output is combined with the others in another Union All. Below the screen shot shows what such a package might look like.
The benefit of this approach is that at the expense of a little more development time, you now have a system that performs efficiently for the most common Lookups and fails over to a slower mode for those items that are less common. That means that the Lookup operation will be extremely efficient for most of your data, which typically results in an overall decrease in processing time.
In other words, you have used the Pareto principle (80/20 rule) to improve the solution. The first (full-cache) Lookup stores 20% of the reference (in this case product) rows and hopefully succeeds in answering 80% of the Lookup requests. This is largely dependent on the user creating the right query to get the proper 20%. If the wrong data is queried then this can be a worst approach. For the 20% of Lookups that fail, they are redirected to — and serviced by — the partial-cache Lookup, which operates against the other 80% of data. Because you are constraining the size of the partial cache, you can ensure you don’t run into any memory limitations — at the extreme, you could even use a no-cache Lookup instead of, or in addition to, the partialcache Lookup.
The final piece to this puzzle is how you identify up front which items occur the most frequently in your domain. If the business does not already keep track of this information, you can derive it by collecting statistics within your packages and saving the results to a temporary location. For instance, each time you load your sales data, you could aggregate the number of sales for each item and write the results to a new table you have created for that purpose. The next time you load the product Lookup Transformation, you join the full Product table to the statistics table and return only those rows whose aggregate count is above a certain threshold. (You could also use the data-mining functionality in SQL Server to derive this information, though the details of that are beyond the scope of Joining Data Topic.)