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CHAPTER 8 BITMAP INDEXES
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DML AND BITMAP INDEXES DON T MIX
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Generally, it is quite disastrous to do any updates to columns with enabled bitmap indexes, or even to do inserts and deletes on a table with enabled bitmap indexes. Concurrent DML operations can easily cause deadlocks, and even small serialized DML operations can cause bitmap indexes to grow explosively. The situation for small, occasional changes has improved in 10g, but for earlier versions, you may find that if you can t disable the bitmap index when you need to apply changes to the data, then creating it with an initial large percentage of empty space (specifically pctfree 67) may stabilize the index at a reasonable level of space wastage.
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Given that the meaning of some of the statistics (and in particular the clustering_factor) is different for bitmap indexes, what effect does that have on the estimated cost of using the index Try a simple column = constant query on each of the columns n6, n5, n4, and n3 in turn. The columns all have the same number of distinct values, so the results we get from running autotrace against the four queries may be very informative. Here are the four execution plans from a 9.2.0.6 system: Execution Plan - (n6: B-tree index on clustered column) ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=54 Card=400 Bytes=5600) 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'T1' (Cost=54 Card=400 Bytes=5600) 2 1 INDEX (RANGE SCAN) OF 'T1_I6' (NON-UNIQUE) (Cost=9 Card=400) Execution Plan - (n5: B-tree index on scattered column) ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=170 Card=400 Bytes=5600) 1 0 TABLE ACCESS (FULL) OF 'T1' (Cost=170 Card=400 Bytes=5600) Execution Plan - (n4: bitmap index on clustered column) ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=114 Card=400 Bytes=5600) 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'T1' (Cost=114 Card=400 Bytes=5600) 2 1 BITMAP CONVERSION (TO ROWIDS) 3 2 BITMAP INDEX (SINGLE VALUE) OF 'T1_I4' Execution Plan - (n3: bitmap index on scattered column) ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=117 Card=400 Bytes=5600) 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'T1' (Cost=117 Card=400 Bytes=5600) 2 1 BITMAP CONVERSION (TO ROWIDS) 3 2 BITMAP INDEX (SINGLE VALUE) OF 'T1_I3' You probably weren t surprised to see that the queries against the columns with B-tree indexes had different execution plans for the clustered data (where the index is considered to be reasonably effective) and the scattered data (where the index is considered to be a waste of space).
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CHAPTER 8 BITMAP INDEXES
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But the costs from the two bitmap indexes are nearly identical, regardless of whether the target data is extremely scattered, or very densely packed. (It is a nuisance that the optimizer doesn t report any cost against bitmap indexes this is a critical bit of useful information that is also missing from the explain plan and can only be found in 10053 trace files.) Of course, looking at the statistics available for the two bitmap indexes, it s not really surprising that the costs given for the two paths are about the same there isn t really anything in the statistics that describes the way that the data is distributed through the table. (You may think the factor of 7 difference in the leaf_block counts should mean something, and there is an element of truth in this, but it s not really a good indicator. In this example, that factor is large because of my artificial 90% free space declaration.) So, where does the calculated cost come from for bitmap indexes I can tell you part of the answer, and approximate another part but this is where my warning about accuracy and bugs starts to have effect.
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The Index Component
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Rerun the queries with event 10053 enabled to show the CBO calculation, and you will find the following details in the trace files (this example is from a 9i system): For the query n3 = 2 bitmap on scattered data --------------------------------------------Access path: index (equal) Index: T1_I3 TABLE: T1 RSC_CPU: 0 RSC_IO: 3 IX_SEL: 4.0000e-002 TB_SEL: 4.0000e-002 ******** Bitmap access path accepted ******** Cost: 117 Cost_io: 117 Cost_cpu: 0.000000 Selectivity: 0.040000 Not believed to be index-only. BEST_CST: 116.54 PATH: 20 Degree: 1
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For the query n4 = 2 bitmap on clustered data --------------------------------------------Access path: index (equal) Index: T1_I4 TABLE: T1 RSC_CPU: 0 RSC_IO: 1 IX_SEL: 4.0000e-002 TB_SEL: 4.0000e-002 ******** Bitmap access path accepted ******** Cost: 114 Cost_io: 114 Cost_cpu: 0.000000 Selectivity: 0.040000 Not believed to be index-only. BEST_CST: 114.34 PATH: 20 Degree: 1 Two important points to notice. First, the best_cst in each case is not a whole number it is reported to two decimal places, and then rounded to the nearest whole number (116.54 went up to 117, 114.34 went down to 114).
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