Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 13 ■ PARTITIONING 583 In the case of a globally partitioned index, note that the number of index partitions may, in fact, be different from the number of table partitions. Since global indexes may be partitioned by range or hash only, you must use local indexes if you wish to have a list or composite partitioned index. The local index will be partitioned using the same scheme as the underlying table. ■Note Hash partitioning of global indexes is a new feature in Oracle 10g Release 1 and later only. You may only globally partition by range in Oracle9i and before. Local Indexes vs. Global Indexes In my experience, most partition implementations in data warehouse systems use local indexes. In an OLTP system, global indexes are much more common, and we’ll see why shortly—it has to do with the need to perform partition elimination on the index structures to maintain the same query response times after partitioning as before partitioning them. ■Note Over the last couple of years, it has become more common to see local indexes used in OLTP systems, as such systems have rapidly grown in size. Local indexes have certain properties that make them the best choice for most data warehouse implementations. They support a more available environment (less downtime), since problems will be isolated to one range or hash of data. Since it can point to many table partitions, a global index, on the other hand, may become a point of failure, rendering all partitions inaccessible to certain queries. Local indexes are more flexible when it comes to partition maintenance operations. If the DBA decides to move a table partition, only the associated local index partition needs to be rebuilt or maintained. With a global index, all index partitions must be rebuilt or maintained in real time. The same is true with sliding window implementations, where old data is aged out of the partition and new data is aged in. No local indexes will be in need of a rebuild, but all global indexes will be either rebuilt or maintained during the partition operation. In some cases, Oracle can take advantage of the fact that the index is locally partitioned with the table and will develop optimized query plans based on that. With global indexes, there is no such relationship between the index and table partitions. Local indexes also facilitate a partition point-in-time recovery operation. If a single partition needs to be recovered to an earlier point in time than the rest of the table for some reason, all locally partitioned indexes can be recovered to that same point in time. All global indexes would need to be rebuilt on this object. This does not mean “avoid global indexes”—in fact, they are vitally important for performance reasons, as you’ll learn shortly—you just need to be aware of the implications of using them.
584 CHAPTER 13 ■ PARTITIONING Local Indexes Oracle makes a distinction between the following two types of local indexes: • Local prefixed indexes: These are indexes whereby the partition keys are on the leading edge of the index definition. For example, if a table is range partitioned on a column named LOAD_DATE, a local prefixed index on that table would have LOAD_DATE as the first column in its column list. • Local nonprefixed indexes: These indexes do not have the partition key on the leading edge of their column list. The index may or may not contain the partition key columns. Both types of indexes are able take advantage of partition elimination, both can support uniqueness (as long as the nonprefixed index includes the partition key), and so on. The fact is that a query that uses a local prefixed index will always allow for index partition elimination, whereas a query that uses a local nonprefixed index might not. This is why local nonprefixed indexes are said to be “slower” by some people—they do not enforce partition elimination (but they do support it). There is nothing inherently better about a local prefixed index as opposed to a local nonprefixed index when that index is used as the initial path to the table in a query. What I mean by that is that if the query can start with “scan an index” as the first step, there isn’t much difference between a prefixed and a nonprefixed index. Partition Elimination Behavior For the query that starts with an index access, whether or not it can eliminate partitions from consideration all really depends on the predicate in your query. A small example will help demonstrate this. The following code creates a table, PARTITIONED_TABLE, that is range partitioned on a numeric column A such that values less than two will be in partition PART_1 and values less than three will be in partition PART_2: ops$tkyte@ORA10G> CREATE TABLE partitioned_table 2 ( a int, 3 b int, 4 data char(20) 5 ) 6 PARTITION BY RANGE (a) 7 ( 8 PARTITION part_1 VALUES LESS THAN(2) tablespace p1, 9 PARTITION part_2 VALUES LESS THAN(3) tablespace p2 10 ) 11 / Table created. We then create both a local prefixed index, LOCAL_PREFIXED, and a local nonprefixed index, LOCAL_NONPREFIXED. Note that the nonprefixed index does not have A on the leading edge of its definition, which is what makes it a nonprefixed index:
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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