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November 2013

When do Oracle Parallel Execution Slaves issue buffered physical reads – Part 2?

In the previous post about in-memory parallel execution I described in which cases the in-mem PX can kick in for your parallel queries.
A few years ago (around Oracle 11.2.0.2 and Exadata X2 release time) I was helping a customer with their migration to Exadata X2. Many of the queries ran way slower on Exadata compared to their old HP Superdome. The Exadata system was configured according to the Oracle’s “best practices”, that included setting the parallel_degree_policy = AUTO.

When do Oracle Parallel Execution Slaves issue buffered physical reads – Part 2?

In the previous post about in-memory parallel execution I described in which cases the in-mem PX can kick in for your parallel queries.

A few years ago (around Oracle 11.2.0.2 and Exadata X2 release time) I was helping a customer with their migration to Exadata X2. Many of the queries ran way slower on Exadata compared to their old HP Superdome. The Exadata system was configured according to the Oracle’s “best practices”, that included setting the parallel_degree_policy = AUTO.

When do Oracle Parallel Execution Slaves issue buffered physical reads – Part 2?

In the previous post about in-memory parallel execution I described in which cases the in-mem PX can kick in for your parallel queries.
A few years ago (around Oracle 11.2.0.2 and Exadata X2 release time) I was helping a customer with their migration to Exadata X2. Many of the queries ran way slower on Exadata compared to their old HP Superdome. The Exadata system was configured according to the Oracle’s “best practices”, that included setting the parallel_degree_policy = AUTO.

When do Oracle Parallel Execution Slaves issue buffered physical reads – Part 2?

In the previous post about in-memory parallel execution I described in which cases the in-mem PX can kick in for your parallel queries.
A few years ago (around Oracle 11.2.0.2 and Exadata X2 release time) I was helping a customer with their migration to Exadata X2. Many of the queries ran way slower on Exadata compared to their old HP Superdome. The Exadata system was configured according to the Oracle’s “best practices”, that included setting the parallel_degree_policy = AUTO.

When do Oracle Parallel Execution Slaves issue buffered physical reads – Part 2?

In the previous post about in-memory parallel execution I described in which cases the in-mem PX can kick in for your parallel queries.
A few years ago (around Oracle 11.2.0.2 and Exadata X2 release time) I was helping a customer with their migration to Exadata X2. Many of the queries ran way slower on Exadata compared to their old HP Superdome. The Exadata system was configured according to the Oracle’s “best practices”, that included setting the parallel_degree_policy = AUTO.

Replicating Tanel’s Script Library

Tanel does offer a zip file with all of his scripts. The zip seems up-to-date now; I started doing this alternative technique awhile ago when the zip file didn’t seem to get updated as quickly as the raw scripts directory.

  mkdir tpt
  cd tpt

wget -r -nH --cut-dirs=2 --no-parent --reject="index.html*" http://blog.tanelpoder.com/files/scripts/

  cd ..

[svn/git] add tpt
[svn/git] commit tpt -m "added Tanel Poder's script library to our script repository"

Please remember that as Tanel says on his own website, “always proofread the scripts and test their effect out in a test environment before running in production.”

Replicating Tanel’s Script Library

Tanel does offer a zip file with all of his scripts. The zip seems up-to-date now; I started doing this alternative technique awhile ago when the zip file didn’t seem to get updated as quickly as the raw scripts directory.

  mkdir tpt
  cd tpt

wget -r -nH --cut-dirs=2 --no-parent --reject="index.html*" http://blog.tanelpoder.com/files/scripts/

  cd ..

[svn/git] add tpt
[svn/git] commit tpt -m "added Tanel Poder's script library to our script repository"

Please remember that as Tanel says on his own website, “always proofread the scripts and test their effect out in a test environment before running in production.”

When do Oracle Parallel Execution Slaves issue buffered physical reads – Part 1?

This post applies both to non-Exadata and Exadata systems.

Before Oracle 11.2 came out, it was true to say that Oracle Parallel Execution slaves always do direct path reads (bypassing buffer cache) when doing full segment scans. This should not be taken simplistically though. Even when you were doing full table scans, then yes the scanning was done with direct path multiblock reads – but if you had to visit other, additional blocks out of the scanning sequence, then these extra IOs were done with regular buffered reads. For example, next row piece fetching of chained rows or or undo block access for CR reads was done with buffered single block reads, or even buffered multiblock reads, if some form of prefetching kicked in.

When do Oracle Parallel Execution Slaves issue buffered physical reads – Part 1?

This post applies both to non-Exadata and Exadata systems.

Before Oracle 11.2 came out, it was true to say that Oracle Parallel Execution slaves always do direct path reads (bypassing buffer cache) when doing full segment scans. This should not be taken simplistically though. Even when you were doing full table scans, then yes the scanning was done with direct path multiblock reads – but if you had to visit other, additional blocks out of the scanning sequence, then these extra IOs were done with regular buffered reads. For example, next row piece fetching of chained rows or or undo block access for CR reads was done with buffered single block reads, or even buffered multiblock reads, if some form of prefetching kicked in.

When do Oracle Parallel Execution Slaves issue buffered physical reads – Part 1?

This post applies both to non-Exadata and Exadata systems.

Before Oracle 11.2 came out, it was true to say that Oracle Parallel Execution slaves always do direct path reads (bypassing buffer cache) when doing full segment scans. This should not be taken simplistically though. Even when you were doing full table scans, then yes the scanning was done with direct path multiblock reads – but if you had to visit other, additional blocks out of the scanning sequence, then these extra IOs were done with regular buffered reads. For example, next row piece fetching of chained rows or or undo block access for CR reads was done with buffered single block reads, or even buffered multiblock reads, if some form of prefetching kicked in.