How to read the oracle explanation plan

Oracle Interpretation Plan is a detailed description of the SQL statement execution process. To get an explanation plan, use the EXPLAIN PLAN command. The Interpretation Plan shows a series of operations, each containing the type of operation, the object name, the number of rows and bytes processed, and the cost. Operations such as TABLE ACCESS FULL, INDEX RANGE SCAN, and NESTED LOOPS show how the query is performed. The cost field identifies performance bottlenecks, and optimization measures such as creating indexes or adjusting predicates can resolve them.

How to interpret Oracle Interpretation Plan

Oracle Interpretation Plan is information provided by the database about the execution plan of SQL statements. It provides in-depth insights into how statements are executed and why they are executed in this way. Understanding how to interpret interpretation plans is critical to optimizing query performance.

Step 1: Get an explanation plan

To get an explanation plan, you can use the EXPLAIN PLAN command:

 <code class="sql">EXPLAIN PLAN FOR <sql statement>;</sql></code>

Step 2: Understand the planning structure

An explanation plan consists of a series of rows, each representing an operation in the query execution. The plan tree is rendered from top to bottom, and the root node represents the main operation of the query.

Step 3: Key fields

Each plan line contains the following key fields:

• Operation: Operation type, such as TABLE ACCESS FULL, INDEX RANGE SCAN
• Object Name: The name of the object participating in the operation, such as a table or index
• Rows: Estimate the number of rows processed in this operation
• Bytes: Estimate the number of bytes processed in this operation
• Cost: Estimated cost of operations

Step 4: Interpret the operation

It is crucial to understand the meaning of each operation:

• TABLE ACCESS FULL: Scan all rows from the table.
• INDEX RANGE SCAN: Use index to scan a subset of the table.
• NESTED LOOPS: Check two tables row by row.
• CARTESIAN PRODUCT: Match all rows in two tables.

Step 5: Identify Performance Bottlenecks

The cost field is the key to identifying performance bottlenecks. High-cost operations indicate the need for optimization.

Step 6: Optimize Query

Once the performance bottleneck is identified, the problem can be solved by optimizing the query. For example, you can create or rebuild the index, adjust the query predicate, or use a different join type.

Example:

Consider a query that contains the following explanation plan:

 <code class="text">Operation | Object Name | Rows | Bytes | Cost -------------------------------------------------------- TABLE ACCESS FULL | t1 | 10000 | 50000 | 10 INDEX RANGE SCAN | t2 | 100 | 1000 | 2 NESTED LOOPS | | 10 | 100 | 1</code>

This plan shows that scanning all rows from Table t1 (TABLE ACCESS FULL) is the most expensive operation. Optimizing queries by creating indexes on t1 can reduce costs and improve performance.

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