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Data Segment Compression

Although UPDATE and DELETE operations can be performed on compressed data in Oracle, the performance impact the compress and uncompress operations have on those statements is yet to be determined.  Oracle recommends that compressed data be highly considered for data warehouse environments because data block compression is highly optimized for direct-load operations

The Oracle Database Performance Tuning (1Z0-033) certification test may have a question on Optimizer Data Compression.  Test candidates should have a basic understanding of:

• The benefits that data compression provides
   

• When to activate data compression

Using Explain Plan to Determine Access Paths

The "EXPLAIN PLAN" statement is used to show the access path the optimizer has chosen for a particular SQL statement.

Before issuing an EXPLAIN PLAN statement, you need to run utlxplan.sql to create a plan table to hold its output. utlxplan.sql can be found in the rdbms/admin directory in most installations. utlxplan.sql creates a table called plan_table, which is owned by the account that executed utlxplan.sql plan_table is the default table into which the EXPLAIN PLAN statement inserts rows describing execution plans.

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