Encoded-vector index (EVI) only access in IBM DB2 for i
Using encoded vector indexes to accelerate the projection of columns returned by an SQL query with DB2 for i
EVI only access
IBM DB2 for i continues to enhance the SQE in many ways. One recent, in November 2015, enhancement delivers a new way that SQE can use an EVI to access the data needed by a query through an index implementation. This new capability is called EVI only access (EOA) where the data can be reached by only accessing an index. This article provides some of the foundational details you'll need to use encoded-vector index only access.
EVIs are a patented technology invented by the IBM Research team and they're unique to IBM DB2 for i. What can an EVI do for you? Indexes are a permanent structure, maintained by the database to provide a fast answer to your question (get the data needed by a query). EVIs can be used as part of selection, where the resultant rows are identified. EVIs used for selection are generally considered more beneficial when the column has low cardinality (number of unique values). In 2012, IBM i 7.1 was enhanced to allow EVIs to include and maintain aggregate detail such as summary, average, and count values over important columns.
With this enhancement to support EOA, EVIs can be used for projection of column values. Projection of column values are the columns found between the SELECT and the FROM clauses in your SQL query. Where should you use EOA? As you might expect, I'll have to respond with "It depends". Anyone answering a performance-related question always takes this safe path. The reality of performance is that it truly does depend upon factors, some of which are client-specific. Like others, I'll lean on the "It depends", but also encourage you to get educated and then spend time evaluating your options.
A QAQQINI query option controls whether or not SQE can consider costing and using EVIs for EOA. The control is named ALLOW_EVI_ONLY_ACCESS, and it can be configured to be set to the following values:
- *DEFAULT – In IBM i 7.1, *NO is the default value. In IBM i 7.2, *YES is the default value.
- *YES – EOA is eligible to be considered by SQE
- *NO – EOA is not eligible to be considered by SQE
As you can see, the default behavior differs between IBM i 7.1 and 7.2. Why is this? Because many IBM i 7.1 users prefer operations and query implementations to remain unchanged, we decided to have the support set to off by default on that release. To override this decision, simply update your favorite QAQQINI file to use ALLOW_EVI_ONLY_ACCESS with *YES or update QUSRSYS/QAQQINI to enable EOA across the entire partition. If you're using IBM i 7.2, you don't need to do anything to enable EOA, other than applying the DB2 PTF Group.
The DB2 PTF Group which included EOA enablement is shown in Table 1. To use EOA, your DB2 PTF Group level needs to match or exceed the levels in the table.
Table 1: DB2 PTF Group levels which enabled EOA
|DB2 for i enhancement||IBM i 7.2||IBM i 7.1|
|EVI Only Access (EOA)||SF99702 Level 9||SF99701 Level 38|
EVI RRN probe
When processing the projects of column values, a table probe operation is used to retrieve a specific row from a table based upon its row number. The row number is provided to the table probe access method by some other operation that generates a row number for the table. With the new EOA capability, the column value can now be retrieved from the EVI using an EVI relative record number (RRN) probe. The EVI RRN probe is an index only access method that is used to provide selected columns by retrieving the value from the EVI instead of using a table probe to access the table. Retrieving the value from the EVI should provide better I/O characteristics than the random I/O associated with a table probe operation.
This access method is used in conjunction with a radix index probe, radix index scan, or EVI probe operation. The radix index probe, radix index scan, or EVI probe operation is used to select the rows and then the RRN of the selected row is used to probe into EVIs to retrieve any selected values that were not provided by the index used for selection. The EVI RRN probe can access multiple EVIs to provide selected values.
Table 2 provides a summary of the new EVI RRN probe with its associated advantages, considerations, examples, and other information.
Table 2: EVI RRN probe attributes
|Data access method||EVI RRN probe|
|Description||The encoded-vector index is quickly probed based on the RRNs provided by the underlying index access.|
|Likely to be used|
|Example SQL statement|
SELECT QUANTITY, ORDERPRIORITY, EXTENDEDPRICE FROM ITEM_FACT WHERE SHIPMODE = 'TRUCK' ORDER BY ORDERDATE DESC LIMIT 50 OFFSET 0;
|Example CREATE INDEX SQL statements|
CREATE ENCODED VECTOR ITEM_FACT_QUANTITY_EVI ON ITEM_FACT ( QUANTITY ASC ) WITH 65537 DISTINCT VALUES; CREATE ENCODED VECTOR ITEM_FACT_ORDERPRIORITY_EVI ON ITEM_FACT ( ORDERPRIORITY ASC ) WITH 65537 DISTINCT VALUES; CREATE ENCODED VECTOR ITEM_FACT_EXTENDEDPRICE_EVI ON ITEM_FACT ( EXTENDEDPRICE ASC ) WITH 65537 DISTINCT VALUES; CREATE INDEX ITEM_FACT_SHIPMODE_RADIX ON ITEM_FACT ( SHIPMODE ASC );
|Database monitor and plan cache record indicating use||A QQRID 3001 index used record for each EVI with QQRCOD = 'I8'|
|SMP parallel enabled||Yes|
|Also referred to as||Table probe, preload|
|Visual Explain icon|
Prior to encoded vector index only access, the recommendation had been to only create EVIs for column with low cardinality (small number of distinct values). This recommendation has now changed. EVI RRN probe can be used for columns with high cardinality (large number of distinct values). However, when creating the EVI, the WITH integer DISTINCT VALUES clause should be used to set the initial width of the EVI vector appropriately and to minimize maintenance time if the database manager needs to use a wider vector. Refer to the CREATE INDEX statement in SQL reference for more details.
Table 3 shows the three database Navigator product options. The information and examples within this article were built using the Navigator product found within IBM i Access for Windows. Some refer to this product as System i Navigator, while others call it IBM i Navigator. In this article, the term IBM i Navigator is used.
Table 3: Database Navigator product overview
|Products||IBM i Access for Windows||IBM i Access Client Solutions||IBM i Navigator|
System i Navigator|
or IBM i Navigator
|ACS||Navigator for i|
|Where does it run?||Windows PC Install||Anywhere Java can be used||
Served from IBM i 7.1 and 7.2
|Recent service level?||
IBM i Access Windows Service Pack|
7.1 – SI57907
IBM HTTP Server for i PTF Group:|
7.2 - SF99713 Level 12
7.1 - SF99368 Level 37
|Best of breed features, for the database user||
Run SQL scripts|
Many other features
Run SQL scripts|
SQL Performance Center
|Webpage to watch||ibm.com/systems/power/software/i/access/windows_sp.html||ibm.com/systems/power/software/i/access/solutions_support.html||www-912.ibm.com/s_dir/SLINE003.NSF/PTFbyNumber/SF99713|
|Next (planned) update||June, 2016||July, 2016 → Version 18.104.22.168||New PTF Group every 2 to3 months|
Index advice for EVI RRN probe plans
Index Advisor may advise EVI indexes for use with the EVI RRN probe plan. To limit excessive index advice, SQE won't consider advising EOA unless there's at least one single column EVI over one or more of the projection columns for the query.
The EVI RRN probe plan advice requires one or more indexes to be created as a dependent set. If any of the EVI indexes are missing, the optimizer won't be able to cost and choose these dependent indexes for implementation of the EVI RRN probe.
The Reason Advised column will have reason code 'I8' if you query QSYS2/SYSIXADV directly. If you use IBM i Navigator to examine the index advice, you'll see something similar to the one shown in Figure 1.
Figure 1. Index advice and EOA
The advice is on a per table basis for the query.
Indexes are advised only when the following statements are true:
- At least one of the columns in the select list of the query needs to have an existing single column EVI.
- The number of columns that do not match to an existing EVI must be less than 20.
- All columns must be eligible to be an index key.
When Index Advisor shows a highly dependent advice, use of the exact match capability from Show Statements to find the query in the plan cache is helpful. Once found, use Visual Explain to discover the dependent index advice specific to that query.
EVI RRN probe plan advice example
In the following example, assume that the following indexes already exist.
- A radix index over the column ORDERDATE
- An EVI index over the column ORDERPRIORITY
SELECT QUANTITY, ORDERPRIORITY, EXTENDEDPRICE FROM ITEM_FACT WHERE SHIPMODE = 'TRUCK' ORDER BY ORDERDATE DESC LIMIT 50 OFFSET 0;
The Index Advisor will advise an EOA style EVI index over all over missing columns: QUANTITY and EXTENDEDPRICE. While EOA style index advice appears in the IBM i Navigator Index Advisor, if you have a specific query, you should use the Index Advisor of Visual Explain to quickly identify the missing indexes.
Figure 2 shows the EVI advice generated for the query, when examining the advice using Visual Explain.
Figure 2. EOA and index advice within Visual Explain
Whenever you are wondering why an existing index was not used by SQE, you can look in two places to gain some additional insight.
- Analyze: If you capture an SQL performance monitor, you can use IBM i Navigator or IBM i Access Client Solutions to analyze the monitor. If you drill into the statements detail for SQE, you can access the Indexes Considered action to see the indexes considered by SQE and a basic explanation of why the index was not used. Figure 3 shows only that reason code 20. A verbose reason might appear in a future IBM i release.
- Visual Explain: The same information about indexes considered can be found under the Information About the Plan Performed section and the List of Indexes Optimized detail.
Figure 4 shows that the EVIs are not used by the optimizer for reason 20.
With EOA support, the indexes considered detail has been extended to include two addition reason codes:
- Reason code 20: The access path cannot be used because Encoded Vector Index does not fit in memory.
User response: Consider using a private pool, as discussed in the next section.
- Reason code 21: The access path cannot be used because not all referenced columns had an Encoded Vector Index.
User response: Create single column EVIs over all referenced columns.
Note: Refer to the QQ1000 column of the QQQ3007 table within IBM Knowledge Center for explanations of reasons 1 to 19.
Figure 3. IBM i Navigator analyze indexes considered
Figure 4. Visual Explain indexes considered and not used
Fair share of memory
Because indexes can be very large, SQE has taken a conservative approach when considering whether to use EOA. If the memory impact for an EOA implementation would exceed the user's fair share of memory, SQE will avoid using EOA.
To mitigate this issue, you can configure a private storage pool and use the QAQQINI MEMORY_PREFERENCE option to direct SQE to use that pool for your query. This is a useful tuning technique for queries beyond EOA, but it is a critical success factor for those who need to use EOA with large data. See the end of this article for a pointer to memory preference detail.
Figure 5 shown a screen capture from Visual Explain containing the query implementation after we have created the advised EVI indexes and mitigated the fair share memory issue.
Figure 5. Visual Explain of an EOA implementation
German football and EOA
These two topics have more in common than you might imagine. To demonstrate how EOA might be used, we need to look no further than the German Bundesliga football league. In this fictional example, we have a very large database table consisting of one row for each fan. The row gives us information such as the fan's name, email address, and of course, the team for which the fan root, cheer and encourage through thick and thin, ups and downs, wins and losses.
The query for this example is quite simple. Find the email addresses of all the fans of the Eintracht club, based in Frankfurt. Figure 5 contains a visual explanation of EOA in action.
We begin our examination at red circle numbered 1 in Figure 5. Using a traditional EVI built over the column FCName (that is the football club name), we are able to efficiently find the rows in the table that matches our selection criteria. The traditional EVI has maintained a symbol table containing the various football club names and the associated symbol table code for that value. Because there are only 18 football clubs in Bundesliga, we can conclude that the cardinality of FCName is very low.
At the red circle numbered 2, we see SQE using the EVI symbol table code (5), scanning the EVI vector and identifying the rows that match 'Eintracht'. Well done SQE, cheers from the crowd!
At the red circle numbered 3, we see EOA appear. The vector scan in the previous step has not only identified the selection rows, but because the vector is constructed in a relative row order, we can use that insight to quickly identify the same rows in vector of any associated EVIs. In the example, such an EVI is built over the Email_Address column. EOA can access column values starting with the EVI vector and moving up to the EVI symbol table. As shown in the example, this is the opposite order of processing, as compared to using EVIs for selection of rows.
At red circle numbered 4, we see that the EVI vector code is used to find the column value in the EVI symbol table. This EOA style EVI could have a very high level of cardinality and still be useful. It is important to understand this point, because EVIs were traditionally targeted to lower cardinality selection of data. The other consideration is that if you know that your EOA style EVI index will have higher cardinality, you should use the
WITH xx DISTINCT VALUES so you can start with a wide EVI vector. While DB2 for i can expand the vector size as cardinality pushes it wider, it is better to start with the widest vector if you anticipate many unique values to appear in the column.
Figure 6. Bundesliga football and EOA
Finally, at red circle numbered 5, our query returns the results by accessing only two encoded-vector indexes. DB2 for i and SQE has used EOA to "ein Tor schießen", which translates to "score a goal"!
The goal of this article is to raise awareness to EOA and provide of the basic knowledge to help you get started. Indexes aren't free, so you probably should not just go ahead and build single column EVIs over the columns of your favorite table. Instead, look for an under-performing query and take EOA for a test drive.
If you're responsible for the indexing strategy, you should consider using a couple of resources to help you along the way.
- QAQQINI memory preference by pool name
- Database performance and query optimization
- BOOM! Another IBM i 7.1 Technology Refresh: (EVI aggregates)
- IBM DB2 for i – indexing methods and strategies white paper
- DB2 for i SQL Performance Workshop