The number of distinct values in each column of a table Data distribution histograms The number of distinct index keys Cardinality (the number of columns with similar values for each column) The minimum and maximum values for each column System statistics, which include I/O characteristics of your system; and CPU statistics, which include CPU speed and other related statistics The key to the CBO s capability to pick the best possible query plan is its capability to correctly estimate the cost of the individual operations of the query plan. These cost estimates are derived from the knowledge about the I/O, CPU, and memory resources needed to execute each operation based on the table and index statistics. The estimates are also based on the operating system statistics that I enumerated earlier, and additional information regarding the operating system performance. The database stores the Optimizer statistics that it collects in its data dictionary. The DBA_ TAB_STATISTICS table shows Optimizer statistics for all the tables in your database. You can also see column statistics by querying the DBA_TAB_COL_STATISTICS view, as shown here: SQL> SELECT column_name, num_distinct FROM dba_tab_col_statistics 2* WHERE table_name='PERSONNEL'; COLUMN_NAME NUM_DISTINCT ------------------------------ -----------PERSON_ID 22058066 UPDATED_DATE 1200586 DATE_OF_BIRTH 32185 LAST_NAME 7281 FIRST_NAME 1729 GENDER 2 HANDICAP_FLAG 1 CREATED_DATE 2480278 MIDDLE_NAME 44477 SQL> As you can see, more than 22 million PERSON_ID numbers are in the PERSONNEL table. However, there are only 7,281 distinct last names and 1,729 distinct first names. Of course, the GENDER column has only two distinct values. The Optimizer takes into account these types of information regarding your table data, before deciding on the best plan of execution for a SQL statement that involves the table s columns.

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Optimizer statistics include both object (table and index) statistics, as well as system statistics. Without accurate system statistics, the Optimizer can t come up with valid cost estimates to evaluate alternate execution plans.

To mimic the behavior of default parameters, you have to implement separate functions that forward to the real implementation: ref class R { void f(int i, int j) { /* ... */ } void f(int i) { f(i, 20); } void f() { f(10, 20); } };

Oracle optimizes the throughput of queries by default. Optimizing throughput means using the fewest resources to process the entire SQL statement. You can also ask Oracle to optimize the response time, which usually means using the fewest resources to get the first (or first n) row(s). For batch jobs, response time for individual SQL statements is less important than the total time it , response time is more critical.

You can use any of the following three modes for the Optimizer with the CBO. The value you set for the OPTIMIZER_MODE initialization parameter is the default mode for the Oracle Optimizer. The rule-based optimizer is a deprecated product, and I don t even mention it here. ALL_ROWS: This is the default Optimizer mode, and it directs Oracle to use the CBO whether you have statistics on any of the tables in a query (derived by you through using the DBMS_STATS package or automatically by the Oracle database) or not, with the express goal of maximizing throughput.

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In the case of all three values for the Optimizer mode discussed here, I state that cost optimization is used regardless of whether there are any statistics on the objects that are being accessed in a query. What this means is that in the absence of any statistics collected with the help of the DBMS_STATS package, Oracle uses dynamic sampling techniques to collect the Optimizer statistics at run time. For certain types of objects, such as external tables and remote tables, Oracle uses simple default values, instead of dynamic sampling, for the Optimizer statistics. For example, Oracle uses a default value of 100 bytes for row length. Similarly, the number of rows in a table is approximated by using the number of storage blocks used by a table and the average row length. However, neither dynamic sampling nor default values give results as good as using comprehensive statistics collected using the DBMS_STATS package. Whether you collect statistics manually, or rely on Oracle s Automatic Optimizer Statistics Collection feature (which uses the DBMS_STATS package internally), Optimizer statistics are collected through the DBMS_STATS package.

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