Download Cost-Based Oracle Fundamentals by Jonathan Lewis PDF
By Jonathan Lewis
The query, "Why isn't really Oracle utilizing my index?" has to be probably the most renowned (or possibly unpopular) questions ever requested at the Oracle support boards. you could have picked precisely the correct columns, you could have them within the excellent order, you will have computed data, you could have checked for null columns—and the optimizer flatly refuses to exploit your index until you trace it. What might be going wrong?
If you may have suffered the disappointment of staring at the optimizer do anything thoroughly strange whilst the simplest execution plan is completely visible, or spent hours or days attempting to make the optimizer do what you will want it to do, then this can be the ebook you wish. you are going to come to grasp how the optimizer thinks, comprehend why it makes errors, and realize the knowledge styles that make it move awry. With this data at your fingertips, you'll shop an immense period of time on designing and trouble-shooting your SQL.
The cost-based optimizer is just a section of code that incorporates a version of ways Oracle databases paintings. through employing this version to the facts approximately your information, the optimizer attempts to successfully convert your question into an executable plan. regrettably, the version cannot be ideal, your information cannot be ideal, and the ensuing execution plan could be faraway from perfect.
In Cost-Based Oracle Fundamentals, the 1st ebook in a chain of 3, Jonathan Lewis—one of the major professionals during this field—describes the main frequent elements of the version, what the optimizer does together with your information, and why issues get it wrong. With this knowledge, you may be capable of repair complete areas of difficulty, not only unmarried SQL statements, by way of adjusting the version or developing extra honest statistics.
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Select /*+ cpu_costing ordered_predicates */ v1, n2, n1 from t1 where v1 = 1 n2 = 18 n1 = 998 and and ; select /*+ cpu_costing ordered_predicates */ v1, n2, n1 from t1 23 24 CHAPTER 2 ■ TABLESCANS where n1 = 998 n2 = 18 v1 = 1 and and ; select /*+ cpu_costing ordered_predicates */ v1, n2, n1 from t1 where v1 = '1' n2 = 18 n1 = 998 and and ; As you would expect, the execution plan in all three cases is a full tablescan, and if you use autotrace to see what’s going on, you will discover only that the cost of the query (in all three cases) is 6.
Table 2-1. 82 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). 84). 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 average single-block read time is 5 milliseconds. • The average multiblock read time is 30 milliseconds. • The typical multiblock read size is 12 blocks. 17 18 CHAPTER 2 ■ TABLESCANS The maxthr and slavethr figures relate to throughput for parallel execution slaves. I believe that the figures somehow control the maximum degree of parallelism that any given query may operate at by recording the maximum rate at which slaves have historically been able to operate—but I have not been able to verify this.