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Spotlight on Oracle 11.0 - Getting Started Guide

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Chains / Migrations Page

Row chaining occurs when a row cannot physically fit into an Oracle block, and another block is required to store the remainder of the row.

Migration of an Oracle row occurs when a row is updated in an Oracle block and the amount of free space in the block is not enough to store all of the row's data. The row is migrated to another physical block in the table, but the indexes that refer to the row still indicate the block where the row used to be.

To open the Chains / Migrations page

  1. Select the Spotlight on Oracle RAC connection in the Spotlight Browser.
  2. Click I/O | Chains / Migrations.

Fetch Rates Chart

Fetch Rates shows the rate of relevant row fetch operations.

  • By rowid indicates the number of rows fetched by ROWID address — this usually occurs when a row is accessed via an index lookup.
  • Continued Fetch indicates the number of these fetches that "continued" into another block because the row had migrated or chained.

Note that rows fetched by table can never chain and hence the rate of rows accessed via full table scan is not shown.

Segments With Chained Rows Grid

Segments with Chained Rows shows tables that are known to have chained or migrated rows.

Chained rows are noted when a table is analyzed using the ANALYZE command. Tables that have not been analyzed or have been analyzed via procedures in the DBMS_STATS package will not be displayed here, they will need to be analyzed with the ANALYZE command.

Column Description

Table Name

The name of the table under investigation.

Rows

The number of rows in the table.

Chained Rows

The number of chained rows in the table.

Pct Chained

The percentage of rows in the table that are chained.

PCTFREE

The minimum percentage of a data block that is kept free to accommodate updates to rows that already exist in the block.

Sessions Experiencing Continued Fetch Grid

Sessions Experiencing Continued Fetch shows sessions that have experienced Continued Fetch operations as they accessed rows. The current SQL statement is also shown.

Column Description
Instance name The instance in the Oracle RAC cluster where the session has experienced Continued Fetch operations.

SID

Session ID.

Oracle User

The Oracle account name for the session.

Continued Fetches

Fetches from chained or migrated rows.

Logical Reads

Logical database reads for the session.

Physical Reads

Physical database reads for the session.

OS User

The operating system user that established the session.

Machine

The machine hosting the database under investigation.

Program

The program causing the Continued Fetch operation.

Status

The status of the session — for example, ACTIVE or INACTIVE.

Type

The type of session — FOREGROUND or BACKGROUND.

SQL

The current SQL statement.

Process ID

Operating system process ID for the client program.

Serial

Serial number of the session. SIDs can be reused after the session disconnects, but the combination of SID and Serial number is always unique.

SID Serial RAC Instance Number  

 

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