Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

CHAPTER 14
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Parallel Execution
Parallel execution, a feature of Oracle Enterprise Edition (it is not available in the Standard
Edition), was first introduced in Oracle version 7.1.6 in 1994. It is the ability to physically
break a large serial task (any DML, or DDL in general) into many smaller bits that may all be
processed simultaneously. Parallel executions in Oracle mimic the real-life processes we see
all of the time. Rarely would you expect to see a single individual build a house; it is far more
common for many teams of people to work concurrently to rapidly assemble the house. In
that way, certain operations can be divided into smaller tasks and performed concurrently.
For example, the plumbing and electrical wiring can take place at the same time to reduce
the total amount of time required for the job as a whole.
Parallel execution in Oracle follows much the same logic. It is often possible for Oracle
to divide a certain large “job” into smaller parts and to perform each part concurrently. For
example, if a full table scan of a large table is required, there is no reason why Oracle cannot
have four parallel sessions, P001–P004, perform the full scan together, with each session reading
a different portion of the table. If the data scanned by P001–P004 needs to be sorted, this
could be carried out by four more parallel sessions, P005–P008, which could ultimately send
the results to an overall coordinating session for the query.
Parallel execution is a tool that, when wielded properly, may increase the response time
of certain operations by orders of magnitude. When it’s wielded as a “fast = true” switch, the
results are typically quite the opposite. In this chapter, the goal is not to explain precisely how
parallel query is implemented in Oracle, the myriad combinations of plans that can result
from parallel operations, and the like. I feel that much of that material is covered quite well in
the Oracle Administrator’s Guide, the Oracle Concepts Guide and, in particular, the Oracle Data
Warehousing Guide. This chapter’s goal is to give you an understanding of what class of problems
parallel execution is and isn’t appropriate for. Specifically, after looking at when to use
parallel execution, we will cover the following:
• Parallel query: The ability to perform a single query using many operating system
processes or threads. Oracle will find operations it can perform in parallel, such as
full table scans or large sorts, and create a query plan to do so.
• Parallel DML (PDML): This is very similar in nature to parallel query, but it is used in
reference to performing modifications (INSERT, UPDATE, DELETE, and MERGE) using parallel
processing. In this chapter, we’ll look at PDML and discuss some of the inherent
limitations associated with it.
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