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

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CHAPTER 12 ■ DATATYPES 555 ROWID/UROWID Types The last datatypes to discuss are the ROWID and UROWID types. A ROWID is the address of a row in a database. Sufficient information is encoded in the ROWID to locate the row on disk, as well as identify the object the ROWID points to (the table and so on). ROWID’s close relative, UROWID, is a universal ROWID and is used for tables, such as IOTs, and tables accessed via gateways to heterogeneous databases that do not have fixed ROWIDs. The UROWID is a representation of the primary key value of the row and, hence, will vary in size depending on the object it points to. Every row in every table has either a ROWID or a UROWID associated with it. They are considered pseudo columns when retrieved from a table, meaning they are not actually stored with the row, but rather are a derived attribute of the row. A ROWID is generated based on the physical location of the row; it is not stored with it. A UROWID is generated based on the row’s primary key, so in a sense it is stored with the row, but not really, as the UROWID does not exist as a discrete column, but rather as a function of the existing columns. It used to be that for rows with ROWIDs (the most common “type” of rows in Oracle; with the exception of rows in IOTs, all rows have ROWIDs), the ROWIDs were immutable. When a row was inserted, it would be associated with a ROWID, an address, and that ROWID would be associated with that row until it was deleted—until it was physically removed from the database. Over time, this is becoming less true, as there are now operations that may cause a row’s ROWID to change, for example: • Updating the partition key of a row in a partitioned table such that the row must move from one partition to another • Using the FLASHBACK table command to restore a database table to a prior point in time • Performing MOVE operations and many partition operations such as splitting or merge partitions • Using the ALTER TABLE SHRINK SPACE command to perform a segment shrink Now, since ROWIDs can change over time (since they are no longer immutable), it is not recommended to physically store them as columns in database tables. That is, using a ROWID as a datatype of a database column is considered a bad practice and should be avoided. The primary key of the row (which should be immutable) should be used instead, and referential integrity can be in place to ensure data integrity is preserved. You cannot do this with the ROWID types—you cannot create a foreign key from a child table to a parent table by ROWID, and you cannot enforce integrity across tables like that. You must use the primary key constraint. Of what use is the ROWID type, then? It is still useful in applications that allow the end user to interact with the data—the ROWID, being a physical address of a row, is the fastest way to access a single row in any table. An application that reads data out of the database and presents it to the end user can use the ROWID upon attempting to update that row. The application must use the ROWID in combination with other fields or checksums (refer to Chapter 7 for further information on application locking). In this fashion, you can update the row in question with the least amount of work (e.g., no index lookup to find the row again) and ensure the row is the same row you read out in the first place by verifying the column values have not changed. So, a ROWID is useful in applications that employ optimistic locking.
556 CHAPTER 12 ■ DATATYPES Summary In this chapter, we examined the 22 basic datatypes provided by Oracle and saw how they are physically stored and what options are available with each. We started with character strings, the most basic of types, and looked into considerations surrounding multibyte characters and raw binary data. Next, we studied the numeric types, including the very precise Oracle NUMBER type and the new floating-point types provided with Oracle 10g and later. We also gave consideration to the legacy LONG and LONG RAW types, concentrating on how we might work around their existence, as the functionality provided by these types falls far short of that provided by the LOB types. Next, we looked at the datatypes capable of storing dates and times. We covered the basics of date arithmetic, a perplexing issue until you’ve seen it demonstrated. Lastly, in the section on DATEs and TIMESTAMPs, we looked at the INTERVAL type and how best to use it. The most detailed part of the chapter from a physical storage perspective was the LOB section. The LOB type is frequently misunderstood by developers and DBAs alike, so the bulk of the section was spent looking at how they are physically implemented as well as certain performance considerations. The last datatype we looked at was the ROWID/UROWID type. For what now should be obvious reasons, you should not use this datatype as a database column, since ROWIDs are not immutable and no integrity constraints could enforce the parent/child relationship. Rather, you want to store primary keys if you need to “point” to another row.
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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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