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CHAPTER 2 TABLESCANS
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cost comes from the full tablescan in line 2, which will pass 10,000 rows and 40,000 bytes up to the sort (aggregate) line, and the sort (aggregate) in line 1 seems to have been free of charge. Before investigating the arithmetic, let s see how we can persuade Oracle to reevaluate the cost: let s change the size of the db_file_multiblock_read_count. alter session set db_file_multiblock_read_count = 16; When we double the value of this parameter, we find the cost of the tablescan drops but doesn t halve. We can automate the process of recording cost against size of db_file_multiblock_read_count, and Table 2-1 shows a few results this will produce (still from Oracle 8i). Note especially the column headed adjusted dbf_mbrc ; I have calculated this as (10,000 / cost), that is, the number of blocks in my table divided by the calculated cost of scanning the table.
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Table 2-1. Effects of Changing the Multiblock Read Count
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db_file_multiblock_read_count
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4 8 16 32 64 128
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Cost
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2,396 1,518 962 610 387 245
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Adjusted dbf_mbrc
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4.17 6.59 10.40 16.39 25.84 40.82
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The adjusted dbf_mbrc is significant, because a little further experimentation shows that the cost of a tablescan when using the traditional costing methods is (blocks below high water mark / adjusted dbf_mbrc). If you do a tablescan of a 23,729 block table with a db_file_multiblock_read_count of 32, then the cost of the tablescan will be reported as ceil(23,729 / 16.39); if you do a tablescan of a 99-block table with a db_file_multiblock_read_count of 64, then the cost of the tablescan will be ceil(99 / 25.84). There will be small rounding errors, of course if you want a more accurate set of figures, create a very large table (or fake it by using the procedure dbms_stats.set_table_stats to claim that your test table is 128,000,000 blocks). It doesn t matter what your standard block size is (although, as you will see, you have to fiddle with the formula when you start working with nonstandard block sizes), there is just one reference set of values that Oracle uses to calculate the cost of a tablescan. The online code suite includes a script called calc_mbrc.sql that can generate the full set of values for the adjusted dbf_mbrc. If you run this script, you will find that there is a point beyond which the adjusted dbf_mbrc will not change. When Oracle starts up, it negotiates with the operating system to find the largest physical read size your operating system will allow, and silently uses that to limit whatever value you set for the db_file_multiblock_read_count.
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CHAPTER 2 TABLESCANS
HIGH WATER MARK
In general, when a table or index segment is first created, space for that segment will be preallocated from a data file but very little of the space will be formatted for use. As data arrives, blocks will be formatted a few at a time. In the simplest setup, Oracle would format the next five blocks from the preallocated space as the need arose, and the object s high water mark (HWM) would be adjusted to show how many blocks had been formatted and were available for use. With the arrival of ASSM in 9i, Oracle formats groups of adjacent blocks (typically 16, it seems) at a time. The high water mark still identifies the highest formatted block in the segment, but ASSM randomizes the allocation slightly, so that unformatted holes (16 blocks, or multiples thereof) can appear in the middle of the object. ASSM also allocates one or two bitmap space management blocks per extent. So the object is larger, and the cost of a tablescan is pushed upwards for large objects the difference is not huge, but there are other side effects to consider, as you will see in later chapters.
Even in this very simple case, it is important to recognize that there can be a difference between the optimizer s calculations and the run-time activity. The adjusted dbf_mbrc is used purely for calculating the cost. At run time Oracle will try to use the value of the db_file_multiblock_read_count parameter to perform a tablescan although odd glitches such as extent boundaries blocks already being cached and therefore not subject to being read usually mean that a serial tablescan will do a fairly random selection of read sizes from one through to the full db_file_multiblock_read_count. (This typical run-time variation is probably the rationale behind the way the adjusted dbf_mbrc gets more pessimistic as the db_file_multiblock_read_count gets larger and larger.) Before moving on, we should take a look at a slightly more complex query: select val, count(*) from t1 group by val ; Execution Plan (8.1.7.4) ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=1540 Card=582 Bytes=2328) 1 0 SORT (GROUP BY) (Cost=1540 Card=582 Bytes=2328) 2 1 TABLE ACCESS (FULL) OF 'T1' (Cost=1518 Card=10000 Bytes=40000) Note in this case that the cost of the tablescan is unchanged (1,518 in line 2), but the total cost of the query is 1,540 (line 1) the extra 22 must be due to the cost of doing the sort (group by). Whether this is a reasonable estimate or not remains to be seen but it does seem a little expensive for sorting just 40KB of data.
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