Archive for the 'CBO'

“alter session force parallel query”, and indexes

This post is a brief discussion about the advantages of activating parallelism by altering the session environment instead of using the alternative ways (hints, DDL). The latter ways are the most popular in my experience, but I have noticed that their popularity is actually due, quite frequently,...
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Xplan: now with “self” measures for row source operations

One of the most useful information that the Oracle kernel attaches to plans in the library cache are measures of various resource consumption figures, such as elapsed time, consistent and current gets, disk reads, etcetera. These can be made available for each plan line (aka “row source...
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CBO: NewDensity for Frequency Histograms,11g-10.2.0.4 (densities part IV)

As we have seen in the previous posts of this series, in 11g a new figure named “NewDensity” has been introduced as a replacement for the “density” column statistic for columns whose histogram has been collected; this change has been backported in 10.2.0.4 also. In the...
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CBO: “NewDensity” replaces “density” in 11g, 10.2.0.4 (densities part III)

In this post we are going to explore and explain the rationale for the formula used by the CBO to compute the “NewDensity” figure that replaces, from 10.2.0.4 onwards, the “density” column statistic in the cardinality estimation formulae for columns with height-balanced (HB)...
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CBO: the formula for the “density” column statistic (densities part II)

In this post we are going to explore and explain the rationale for the formula used by dbms_stats to compute the “density” column statistic, used by the CBO in versions less than 10.2.0.4 to estimate the cardinality of a class of SQL statements. In the next post, we will speak about its...
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