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

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CHAPTER 4 ■ MEMORY STRUCTURES 121 then it shrunk back down. To perform our query and the sorting, Oracle allocated a sort area for our session. Additionally, the PGA memory went from 487KB to 551KB, a jump of 64KB. Also, we can see that we did 2,906 writes and reads to and from temp. By the time we finish our query and exhausted the resultset, we can see that our UGA memory went back down where it started (we released the sort areas from our UGA) and the PGA shrunk back somewhat (note that in Oracle8i and before, you would not expect to see the PGA shrink back at all; this is new with Oracle9i and later). Let’s retry that operation but play around with the size of our SORT_AREA_SIZE increasing it to 1MB. We’ll log out of the session we’re monitoring, log back in, and use the reset_stat.sql script to start over. As the beginning numbers are consistent, I don’t display them here—only the final results: ops$tkyte@ORA10G> alter session set sort_area_size=1048576; Session altered. ops$tkyte@ORA10G> set termout off; query was executed here ops$tkyte@ORA10G> set termout on Now in the other session we can measure our memory usage again: ops$tkyte@ORA10G> @watch_stat 6 rows merged. NAME VALUE DIFF ------------------------------------------- ---------- ---------- physical reads direct temporary tablespace 684 684 physical writes direct temporary tablespace 684 684 session pga memory 498252 0 session pga memory max 2398796 1900544 session uga memory 152176 0 session uga memory max 1265064 1112888 6 rows selected. As you can see, our PGA had grown considerably this time during the processing of our query. It temporarily grew by about 1,728KB, but the amount of physical I/O we had to do to sort this data dropped considerably as well (use more memory, swap to disk less often). We may have avoided a multipass sort as well, a condition that happens when there are so many little sets of sorted data to merge together that Oracle ends up writing the data to temp more than once. Now, let’s go to an extreme here: ops$tkyte@ORA10G> alter session set sort_area_size=1073741820; Session altered. ops$tkyte@ORA10G> set termout off; query was executed here ops$tkyte@ORA10G> set termout on
122 CHAPTER 4 ■ MEMORY STRUCTURES Measuring from the other session, we can see the memory used so far: ops$tkyte@ORA10G> @watch_stat 6 rows merged. NAME VALUE DIFF ------------------------------------------- ---------- ---------- physical reads direct temporary tablespace 0 0 physical writes direct temporary tablespace 0 0 session pga memory 498252 0 session pga memory max 7445068 6946816 session uga memory 152176 0 session uga memory max 7091360 6939184 6 rows selected. We can observe that even though we allowed for up to 1GB of memory to the SORT_AREA_ SIZE, we really only used about 6.6MB. This shows that the SORT_AREA_SIZE setting is an upper bound, not the default and only allocation size. Here notice also that we did only one sort again, but this time it was entirely in memory; there was no temporary space on disk used, as evidenced by the lack of physical I/O. If you run this same test on various versions of Oracle, or perhaps even on different operating systems, you might see different behavior, and I would expect that your numbers in all cases would be a little different from mine. But the general behavior should be the same. In other words, as you increase the permitted sort area size and perform large sorts, the amount of memory used by your session will increase. You might notice the PGA memory going up and down, or it might remain constant over time, as just shown. For example, if you were to execute the previous test in Oracle8i, I am sure that you would notice that PGA memory does not shrink back in size (i.e., the SESSION PGA MEMORY equals the SESSION PGA MEMORY MAX in all cases). This is to be expected, as the PGA is managed as a heap in 8i releases and is created via malloc()-ed memory. In 9i and 10g, new methods attach and release work areas as needed using operating system–specific memory allocation calls. The important things to remember about using the *_AREA_SIZE parameters are as follows: • These parameters control the maximum amount of memory used by a SORT, HASH, and/or BITMAP MERGE operation. • A single query may have many operations taking place that use this memory, and multiple sort/hash areas could be created. Remember that you may have many cursors opened simultaneously, each with their own SORT_AREA_RETAINED needs. So, if you set the sort area size to 10MB, you could use 10, 100, 1,000 or more megabytes of RAM in your session. These settings are not session limits; rather, they are limits on a single operation, and your session could have many sorts in a single query or many queries open that require a sort.
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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■CONTENTS vii ■CHAPTER 9 Redo a
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■CONTENTS ix ■CHAPTER 12 Dataty
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Foreword “THINK.” In 1914, Thom
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■FOREWORD xiii Tom is an aficiona
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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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■INTRODUCTION xix • Exposure to
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■INTRODUCTION xxi Chapter 3: File
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■INTRODUCTION xxiii Next up are t
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Setting Up Your Environment In this
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CHAPTER 1 ■ ■ ■ Developing Su
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CHAPTER 1 ■ DEVELOPING SUCCESSFUL
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CHAPTER 6 ■ ■ ■ Locking and L
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CHAPTER 6 ■ LOCKING AND LATCHING
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284 CHAPTER 9 ■ REDO AND UNDO cri
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 11 ■ INDEXES 431 The data
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CHAPTER 11 ■ INDEXES 445 Indicate
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CHAPTER 13 ■ ■ ■ Partitioning
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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