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Figure 10-6. IOT with overflow segment, PCTTHRESHOLD clause The other option is to use the INCLUDING clause. Here we are stating explicitly what columns we want stored on the index block and which should be stored in the overflow. Given a CREATE TABLE like this: ops$tkyte@ORA10GR1> create table iot 2 ( x int, 3 y date, 4 z varchar2(2000), 5 constraint iot_pk primary key (x) 6 ) 7 organization index 8 including y 9 overflow 10 / Table created. what we can expect to find is illustrated in Figure 10-7.
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Figure 10-7. IOT with OVERFLOW segment, INCLUDING clause
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CHAPTER 10 DATABASE TABLES
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In this situation, regardless of the size of the data stored in it, Z will be stored out of line in the overflow segment. Which is better, then: PCTTHRESHOLD, INCLUDING, or some combination of both It depends on your needs. If you have an application that always, or almost always, uses the first four columns of a table and rarely accesses the last five columns, using INCLUDING would be appropriate. You would include up to the fourth column and let the other five be stored out of line. At runtime, if you need them, the columns will be retrieved in much the same way as a migrated or chained row would be. Oracle will read the head of the row, find the pointer to the rest of the row, and then read that. If, on the other hand, you cannot say that you almost always access these columns and hardly ever access those columns, you should give some consideration to PCTTHRESHOLD. Setting PCTTHRESHOLD is easy once you determine the number of rows you would like to store per index block on average. Suppose you wanted 20 rows per index block. Well, that means each row should be one-twentieth (5 percent). Your PCTTHRESHOLD would be 5, and each chunk of the row that stays on the index leaf block should consume no more than 5 percent of the block. The last thing to consider with IOTs is indexing. You can have an index on an IOT itself sort of like having an index on an index. These are called secondary indexes. Normally, an index contains the physical address of the row it points to, the rowid. An IOT secondary index cannot do this; it must use some other way to address the row. This is because a row in an IOT can move around a lot, and it does not migrate in the way a row in a heap organized table would. A row in an IOT is expected to be at some position in the index structure, based on its primary key value; it will only be moving because the size and shape of the index itself is changing. (We ll cover more about how index structures are maintained in the next chapter.) To accommodate this, Oracle introduced a logical rowid. These logical rowids are based on the IOT s primary key. They may also contain a guess as to the current location of the row, although this guess is almost always wrong because after a short while, data in an IOT tends to move. The guess is the physical address of the row in the IOT when it was first placed into the secondary index structure. If the row in the IOT has to move to another block, the guess in the secondary index becomes stale. Therefore, an index on an IOT is slightly less efficient than an index on a regular table. On a regular table, an index access typically requires the I/O to scan the index structure and then a single read to read the table data. With an IOT, typically two scans are performed: one on the secondary structure and the other on the IOT itself. That aside, indexes on IOTs provide fast and efficient access to the data in the IOT using columns other than the primary key.
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