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

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CHAPTER 11 ■ INDEXES 425 One of the properties of a B*Tree is that all leaf blocks should be at the same level in the tree. This level is also known as the height of the index, meaning that any traversal from the root block of the index to a leaf block will visit the same number of blocks. That is, to get to the leaf block to retrieve the first row for a query of the form "SELECT INDEXED_COL FROM T WHERE ➥ INDEXED_COL = :X" will take the same number of I/Os regardless of the value of :X that is used. In other words, the index is height balanced. Most B*Tree indexes will have a height of 2 or 3, even for millions of records. This means that it will take, in general, two or three I/Os to find your key in the index—which is not too bad. ■Note Oracle uses two terms with slightly different meanings when referring to the number of blocks involved in traversing from an index root block to a leaf block. The first is HEIGHT, which is the number of blocks required to go from the root block to the leaf block. The HEIGHT value can be found from the INDEX_ STATS view after the index has been analyzed using the ANALYZE INDEX VALIDATE STRUCTURE command. The other is BLEVEL, which is the number of branch levels and differs from HEIGHT by one (it does not count the leaf blocks). The value of BLEVEL is found in the normal dictionary tables such as USER_INDEXES after statistics have been gathered. For example, say we have a 10,000,000-row table with a primary key index on a number column: big_table@ORA9IR2> select index_name, blevel, num_rows 2 from user_indexes 3 where table_name = 'BIG_TABLE'; INDEX_NAME BLEVEL NUM_ROWS ------------------------------ ---------- ---------- BIG_TABLE_PK 2 10441513 The BLEVEL is 2, meaning the HEIGHT is 3, and it will take two I/Os to find a leaf (resulting in a third I/O). So we would expect three I/Os to retrieve any given key value from this index: big_table@ORA9IR2> select id from big_table where id = 42; Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=6) 1 0 INDEX (UNIQUE SCAN) OF 'BIG_TABLE_PK' (UNIQUE) (Cost=2 Card=1 Bytes=6) Statistics ---------------------------------------------------------- ... 3 consistent gets ... 1 rows processed
426 CHAPTER 11 ■ INDEXES big_table@ORA9IR2> select id from big_table where id = 12345; Statistics ---------------------------------------------------------- ... 3 consistent gets ... 1 rows processed big_table@ORA9IR2> select id from big_table where id = 1234567; Statistics ---------------------------------------------------------- ... 3 consistent gets ... 1 rows processed The B*Tree is an excellent general-purpose indexing mechanism that works well for large and small tables, and experiences little, if any, degradation in retrieval performance as the size of the underlying table grows. Index Key Compression One of the interesting things you can do with a B*Tree index is “compress” it. This is not compression in the same manner that ZIP files are compressed; rather, this is compression that removes redundancies from concatenated (multicolumn) indexes. We covered compressed key indexes in some detail in the section “Index Organized Tables” in Chapter 10, and we will take a brief look at them again here. The basic concept behind a compressed key index is that every entry is broken into two pieces: a “prefix” and “suffix” component. The prefix is built on the leading columns of the concatenated index and will have many repeating values. The suffix is built on the trailing columns in the index key and is the unique component of the index entry within the prefix. By way of example, we’ll create a table and a concatenated index and measure its space without compression using ANALYZE INDEX. We’ll then re-create the index with index key compression, compressing a different number of key entries, and see the difference. Let’s start with this table and index: ops$tkyte@ORA10G> create table t 2 as 3 select * from all_objects; Table created. ops$tkyte@ORA10G> create index t_idx on 2 t(owner,object_type,object_name); Index created. ops$tkyte@ORA10G> analyze index t_idx validate structure; Index analyzed.
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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 xxiii Next up are t
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Setting Up Your Environment In this
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CHAPTER 13 ■ ■ ■ Partitioning
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