|Oracle® Database Concepts
10g Release 2 (10.2)
|PDF · Mobi · ePub|
This chapter discusses the Oracle built-in datatypes, their properties, and how they map to non-Oracle datatypes.
This chapter contains the following topics:
Each column value and constant in a SQL statement has a datatype, which is associated with a specific storage format, constraints, and a valid range of values. When you create a table, you must specify a datatype for each of its columns.
Oracle Database PL/SQL User's Guide and Reference for more information about PL/SQL datatypes
Oracle Database Application Developer's Guide - Fundamentals for information about how to use the built-in datatypes
The following sections that describe each of the built-in datatypes in more detail.
The database's character set is established when you create the database. Examples of character sets are 7-bit ASCII (American Standard Code for Information Interchange), EBCDIC (Extended Binary Coded Decimal Interchange Code), Code Page 500, Japan Extended UNIX, and Unicode UTF-8. Oracle supports both single-byte and multibyte encoding schemes.
Oracle Database Application Developer's Guide - Fundamentals for information about how to select a character datatype
Oracle Database Globalization Support Guide for more information about converting character data
CHAR datatype stores fixed-length character strings. When you create a table with a
CHAR column, you must specify a string length (in bytes or characters) between 1 and 2000 bytes for the
CHAR column width. The default is 1 byte. Oracle then guarantees that:
When you insert or update a row in the table, the value for the
CHAR column has the fixed length.
If you give a shorter value, then the value is blank-padded to the fixed length.
If a value is too large, Oracle returns an error.
See Also:Oracle Database SQL Reference for details about blank-padded comparison semantics
VARCHAR2 datatype stores variable-length character strings. When you create a table with a
VARCHAR2 column, you specify a maximum string length (in bytes or characters) between 1 and 4000 bytes for the
VARCHAR2 column. For each row, Oracle stores each value in the column as a variable-length field unless a value exceeds the column's maximum length, in which case Oracle returns an error. Using
VARCHAR saves on space used by the table.
For example, assume you declare a column
VARCHAR2 with a maximum size of 50 characters. In a single-byte character set, if only 10 characters are given for the
VARCHAR2 column value in a particular row, the column in the row's row piece stores only the 10 characters (10 bytes), not 50.
See Also:Oracle Database SQL Reference for details about nonpadded comparison semantics
Globalization support allows the use of various character sets for the character datatypes. Globalization support lets you process single-byte and multibyte character data and convert between character sets. Client sessions can use client character sets that are different from the database character set.
Consider the size of characters when you specify the column length for character datatypes. You must consider this issue when estimating space for tables with columns that contain character data.
The length semantics of character datatypes can be measured in bytes or characters.
For single byte character sets, columns defined in character semantics are basically the same as those defined in byte semantics. Character semantics are useful for defining varying-width multibyte strings; it reduces the complexity when defining the actual length requirements for data storage. For example, in a Unicode database (
UTF8), you need to define a
VARCHAR2 column that can store up to five Chinese characters together with five English characters. In byte semantics, this would require (5*3 bytes) + (1*5 bytes) = 20 bytes; in character semantics, the column would require 10 characters.
VARCHAR2(20 BYTE) and
20) use byte semantics.
VARCHAR2(10 CHAR) and
10) use character semantics.
NLS_LENGTH_SEMANTICS decides whether a new column of character datatype uses byte or character semantics. The default length semantic is byte. If all character datatype columns in a database use byte semantics (or all use character semantics) then users do not have to worry about which columns use which semantics. The
CHAR qualifiers shown earlier should be avoided when possible, because they lead to mixed-semantics databases. Instead, the
NLS_LENGTH_SEMANTICS initialization parameter should be set appropriately in the server parameter file (SPFILE) or initialization parameter file, and columns should use the default semantics.
Oracle Database Globalization Support Guide for information about Oracle's globalization support feature
Oracle Database Application Developer's Guide - Fundamentals for information about setting length semantics and choosing the appropriate Unicode character set.
Oracle Database Upgrade Guide for information about migrating existing columns to character semantics
NVARCHAR2 are Unicode datatypes that store Unicode character data. The character set of
NVARCHAR2 datatypes can only be either
UTF8 and is specified at database creation time as the national character set.
UTF8 are both Unicode encoding.
NCHAR datatype stores fixed-length character strings that correspond to the national character set.
NVARCHAR2 datatype stores variable length character strings.
When you create a table with an
NVARCHAR2 column, the maximum size specified is always in character length semantics. Character length semantics is the default and only length semantics for
For example, if national character set is
UTF8, then the following statement defines the maximum byte length of 90 bytes:
CREATE TABLE tab1 (col1 NCHAR(30));
This statement creates a column with maximum character length of 30. The maximum byte length is the multiple of the maximum character length and the maximum number of bytes in each character.
The maximum length of an
NCHAR column is 2000 bytes. It can hold up to 2000 characters. The actual data is subject to the maximum byte limit of 2000. The two size constraints must be satisfied simultaneously at run time.
The maximum length of an
NVARCHAR2 column is 4000 bytes. It can hold up to 4000 characters. The actual data is subject to the maximum byte limit of 4000. The two size constraints must be satisfied simultaneously at run time.
See Also:Oracle Database Globalization Support Guide for more information about the
Unicode is an effort to have a unified encoding of every character in every language known to man. It also provides a way to represent privately-defined characters. A database column that stores Unicode can store text written in any language.
Oracle users deploying globalized applications have a strong need to store Unicode data in Oracle databases. They need a datatype which is guaranteed to be Unicode regardless of the database character set.
Oracle supports a reliable Unicode datatype through
NCLOB. These datatypes are guaranteed to be Unicode encoding and always use character length semantics. The character sets used by
NCHAR/NVARCHAR2 can be either
AL16UTF16, depending on the setting of the national character set when the database is created. These datatypes allow character data in Unicode to be stored in a database that may or may not use Unicode as database character set.
The LOB datatypes for character data are
NCLOB. They can store up to 8 terabytes of character data (
CLOB) or national character set data (
See Also:"Overview of LOB Datatypes"
Note:Do not create tables with
LONGcolumns. Use LOB columns (
LONGcolumns are supported only for backward compatibility.
Oracle also recommends that you convert existing
LONG columns to LOB columns. LOB columns are subject to far fewer restrictions than
LONG columns. Further, LOB functionality is enhanced in every release, whereas
LONG functionality has been static for several releases.
Columns defined as
LONG can store variable-length character data containing up to 2 gigabytes of information.
LONG data is text data that is to be appropriately converted when moving among different systems.
LONG datatype columns are used in the data dictionary to store the text of view definitions. You can use
LONG columns in
SET clauses of
UPDATE statements, and
VALUES clauses of
Oracle Database Application Developer's Guide - Fundamentals for information about the restrictions on the
"Overview of RAW and LONG RAW Datatypes" for information about the
The numeric datatypes store positive and negative fixed and floating-point numbers, zero, infinity, and values that are the undefined result of an operation (that is, is "not a number" or NAN).
NUMBER datatype stores fixed and floating-point numbers. Numbers of virtually any magnitude can be stored and are guaranteed portable among different systems operating Oracle, up to 38 digits of precision.
The following numbers can be stored in a
Positive numbers in the range 1 x 10-130 to 9.99...9 x 10125 with up to 38 significant digits
Negative numbers from -1 x 10-130 to 9.99...99 x 10125 with up to 38 significant digits
Positive and negative infinity (generated only by importing from an Oracle Version 5 database)
For numeric columns, you can specify the column as:
Optionally, you can also specify a precision (total number of digits) and scale (number of digits to the right of the decimal point):
column_name NUMBER (precision, scale)
If a precision is not specified, the column stores values as given. If no scale is specified, the scale is zero.
Oracle guarantees portability of numbers with a precision equal to or less than 38 digits. You can specify a scale and no precision:
column_name NUMBER (*, scale)
In this case, the precision is 38, and the specified scale is maintained.
When you specify numeric fields, it is a good idea to specify the precision and scale. This provides extra integrity checking on input.
Table 26-1 shows examples of how data would be stored using different scale factors.
|Input Data||Specified As||Stored As|
(not accepted, exceeds precision)
If you specify a negative scale, then Oracle rounds the actual data to the specified number of places to the left of the decimal point. For example, specifying (7,-2) means Oracle rounds to the nearest hundredths, as shown in Table 26-1.
For input and output of numbers, the standard Oracle default decimal character is a period, as in the number 1234.56. The decimal is the character that separates the integer and decimal parts of a number. You can change the default decimal character with the initialization parameter
NLS_NUMERIC_CHARACTERS. You can also change it for the duration of a session with the
ALTER SESSION statement. To enter numbers that do not use the current default decimal character, use the
Oracle stores numeric data in variable-length format. Each value is stored in scientific notation, with 1 byte used to store the exponent and up to 20 bytes to store the mantissa. The resulting value is limited to 38 digits of precision. Oracle does not store leading and trailing zeros. For example, the number 412 is stored in a format similar to 4.12 x 102, with 1 byte used to store the exponent(
2) and 2 bytes used to store the three significant digits of the mantissa(
4,1,2). Negative numbers include the sign in their length.
Taking this into account, the column size in bytes for a particular numeric data value
p is the precision of a given value, can be calculated using the following formula:
s equals zero if the number is positive, and
s equals 1 if the number is negative.
Zero and positive and negative infinity (only generated on import from Version 5 Oracle databases) are stored using unique representations. Zero and negative infinity each require 1 byte; positive infinity requires 2 bytes.
Oracle provides two numeric datatypes exclusively for floating-point numbers:
BINARY_DOUBLE. They support all of the basic functionality provided by the
NUMBER datatype. However, while
NUMBER uses decimal precision,
BINARY_DOUBLE use binary precision. This enables faster arithmetic calculations and usually reduces storage requirements.
BINARY_DOUBLE are approximate numeric datatypes. They store approximate representations of decimal values, rather than exact representations. For example, the value 0.1 cannot be exactly represented by either
BINARY_FLOAT. They are frequently used for scientific computations. Their behavior is similar to the datatypes
DOUBLE in Java and XMLSchema.
BINARY_FLOATimplement most of the Institute of Electrical and Electronics Engineers (IEEE) Standard for Binary Floating-Point Arithmetic, IEEE Standard 754-1985 (IEEE754). For a full description of the Oracle implementation of floating-point numbers and its differences from IEEE754, see the Oracle Database SQL Reference
DATE datatype stores point-in-time values (dates and times) in a table. The
DATE datatype stores the year (including the century), the month, the day, the hours, the minutes, and the seconds (after midnight).
Oracle can store dates in the Julian era, ranging from January 1, 4712 BCE through December 31, 4712 CE (Common Era, or 'AD'). Unless BCE ('BC' in the format mask) is specifically used, CE date entries are the default.
Oracle uses its own internal format to store dates. Date data is stored in fixed-length fields of seven bytes each, corresponding to century, year, month, day, hour, minute, and second.
For input and output of dates, the standard Oracle date format is
DD-MON-YY, as follows:
You can change this default date format for an instance with the parameter
NLS_DATE_FORMAT. You can also change it during a user session with the
ALTER SESSION statement. To enter dates that are not in standard Oracle date format, use the
TO_DATE function with a format mask:
TO_DATE ('November 13, 1992', 'MONTH DD, YYYY')
Oracle stores time in 24-hour format—
By default, the time in a date field is
00:00:00 A.M. (midnight) if no time portion is entered. In a time-only entry, the date portion defaults to the first day of the current month. To enter the time portion of a date, use the
TO_DATE function with a format mask indicating the time portion, as in:
INSERT INTO birthdays (bname, bday) VALUES ('ANDY',TO_DATE('13-AUG-66 12:56 A.M.','DD-MON-YY HH:MI A.M.'));
Julian dates allow continuous dating by the number of days from a common reference. (The reference is 01-01-4712 years BCE, so current dates are somewhere in the 2.4 million range.) A Julian date is nominally a noninteger, the fractional part being a portion of a day. Oracle uses a simplified approach that results in integer values. Julian dates can be calculated and interpreted differently. The calculation method used by Oracle results in a seven-digit number (for dates most often used), such as 2449086 for 08-APR-93.
Note:Oracle Julian dates might not be compatible with Julian dates generated by other date algorithms.
SELECT TO_CHAR (hire_date, 'J') FROM employees;
INSERT INTO employees (hire_date) VALUES (TO_DATE(2448921, 'J'));
Oracle date arithmetic takes into account the anomalies of the calendars used throughout history. For example, the switch from the Julian to the Gregorian calendar, 15-10-1582, eliminated the previous 10 days (05-10-1582 through 14-10-1582). The year 0 does not exist.
You can enter missing dates into the database, but they are ignored in date arithmetic and treated as the next "real" date. For example, the next day after 04-10-1582 is 15-10-1582, and the day following 05-10-1582 is also 15-10-1582.
Note:This discussion of date arithmetic might not apply to all countries' date standards (such as those in Asia).
Oracle stores year data with the century information. For example, the Oracle database stores 1996 or 2001, and not simply 96 or 01. The
DATE datatype always stores a four-digit year internally, and all other dates stored internally in the database have four digit years. Oracle utilities such as import, export, and recovery also deal with four-digit years.
Oracle Database provides daylight savings support for
DATETIME datatypes in the server. You can insert and query
DATETIME values based on local time in a specific region. The
ZONE are time-zone aware.
Oracle Database Application Developer's Guide - Fundamentals for more information about centuries and date format masks
Oracle Database SQL Reference for information about date format codes
|Datatype||Time Zone||Fractional Seconds|
ZONE is stored in the database time zone. When a user selects the data, the value is adjusted to the user's session time zone.
For example, a San Francisco database has system time zone = -8:00. When a New York client (session time zone = -5:00) inserts into or selects from the San Francisco database,
ZONE data is adjusted as follows:
The New York client inserts
TIMESTAMP'1998-1-23 6:00:00-5:00' into a
ZONE column in the San Francisco database. The inserted data is stored in San Francisco as binary value
When the New York client selects that inserted data from the San Francisco database, the value displayed in New York is
A San Francisco client, selecting the same data, see the value
Note:To avoid unexpected results in your DML operations on datatime data, you can verify the database and session time zones by querying the built-in SQL functions
SESSIONTIMEZONE. If the database time zone or the session time zone has not been set manually, Oracle uses the operating system time zone by default. If the operating system time zone is not a valid Oracle time zone, then Oracle uses UTC as the default value.
See Also:Oracle Database SQL Reference for details about the syntax of creating and entering data in time stamp columns
The LOB datatypes
BFILE enable you to store and manipulate large blocks of unstructured data (such as text, graphic images, video clips, and sound waveforms) in binary or character format. They provide efficient, random, piece-wise access to the data. Oracle recommends that you always use LOB datatypes over
LONG datatypes. You can perform parallel queries (but not parallel DML or DDL) on LOB columns.
LOB datatypes differ from
RAW datatypes in several ways. For example:
A table can contain multiple LOB columns but only one
A table containing one or more LOB columns can be partitioned, but a table containing a
LONG column cannot be partitioned.
The maximum size of a LOB is 8 terabytes, and the maximum size of a
LONG is only 2 gigabytes.
LOBs support random access to data, but
LONGs support only sequential access.
LOB datatypes (except
NCLOB) can be attributes of a user-defined object type but
LONG datatypes cannot.
Temporary LOBs that act like local variables can be used to perform transformations on LOB data. Temporary internal LOBs (
NCLOBs) are created in a temporary tablespace and are independent of tables. For
LONG datatypes, however, no temporary structures are available.
Tables with LOB columns can be replicated, but tables with
LONG columns cannot.
SQL statements define LOB columns in a table and LOB attributes in a user-defined object type. When defining LOBs in a table, you can explicitly specify the tablespace and storage characteristics for each LOB.
LOB datatypes can be stored inline (within a table), out-of-line (within a tablespace, using a LOB locator), or in an external file (
BFILE datatypes). With compatibility set to Oracle9i or higher, you can use LOBs with SQL
VARCHAR operators and functions.
Oracle Database SQL Reference for a list of differences between the LOB datatypes and the
Oracle Database Application Developer's Guide - Large Objects for more information about LOB storage and LOB locators
BLOBs participate fully in transactions. Changes made to a
BLOB value by the
DBMS_LOB package, PL/SQL, or the OCI can be committed or rolled back. However,
BLOB locators cannot span transactions or sessions.
NCLOB datatypes store up to 8 terabytes of character data in the database.
CLOBs store database character set data, and
NCLOBs store Unicode national character set data. Storing varying-width LOB data in a fixed-width Unicode character set internally enables Oracle to provide efficient character-based random access on CLOBs and NCLOBs.
NCLOBs participate fully in transactions. Changes made to a
NCLOB value by the
DBMS_LOB package, PL/SQL, or the OCI can be committed or rolled back. However,
NCLOB locators cannot span transactions or sessions. You cannot create an object type with
NCLOB attributes, but you can specify
NCLOB parameters in a method for an object type.
See Also:Oracle Database Globalization Support Guide for more information about national character set data and Unicode
BFILE datatype stores unstructured binary data in operating-system files outside the database. A
BFILE column or attribute stores a file locator that points to an external file containing the data.
BFILEs can store up to 8 terabytes of data.
BFILEs are read only; you cannot modify them. They support only random (not sequential) reads, and they do not participate in transactions. The underlying operating system must maintain the file integrity, security, and durability for
BFILEs. The database administrator must ensure that the file exists and that Oracle processes have operating-system read permissions on the file.
RAWdatatype is provided for backward compatibility with existing applications. For new applications, use the
BFILEdatatypes for large amounts of binary data.
Oracle also recommends that you convert existing
RAW columns to LOB columns. LOB columns are subject to far fewer restrictions than
LONG columns. Further, LOB functionality is enhanced in every release, whereas
RAW functionality has been static for several releases.
RAW datatypes are used for data that is not to be interpreted (not converted when moving data between different systems) by Oracle. These datatypes are intended for binary data or byte strings. For example,
RAW can be used to store graphics, sound, documents, or arrays of binary data. The interpretation depends on the use.
RAW is a variable-length datatype like the
VARCHAR2 character datatype, except Oracle Net Services (which connects user sessions to the instance) and the Import and Export utilities do not perform character conversion when transmitting
RAW data. In contrast, Oracle Net Services and Import/Export automatically convert
LONG data between the database character set and the user session character set, if the two character sets are different.
When Oracle automatically converts
RAW data to and from
CHAR data, the binary data is represented in hexadecimal form with one hexadecimal character representing every four bits of
RAW data. For example, one byte of
RAW data with bits 11001011 is displayed and entered as
See Also:Oracle Database Application Developer's Guide - Fundamentals for information about other restrictions on the
Physical rowids store the addresses of rows in ordinary tables (excluding index-organized tables), clustered tables, table partitions and subpartitions, indexes, and index partitions and subpartitions.
A column of the
UROWID datatype can store all kinds of rowids. The value of the
COMPATIBLE initialization parameter (for file format compatibility) must be set to 8.1 or higher to use
See Also:"Rowids in Non-Oracle Databases"
Each table in an Oracle database internally has a pseudocolumn named
ROWID. This pseudocolumn is not evident when listing the structure of a table by executing a
FROM ... statement, or a
DESCRIBE ... statement using SQL*Plus, nor does the pseudocolumn take up space in the table. However, each row's address can be retrieved with a SQL query using the reserved word
ROWID as a column name, for example:
SELECT ROWID, last_name FROM employees;
You cannot set the value of the pseudocolumn
UPDATE statements, and you cannot delete a
ROWID value. Oracle uses the
ROWID values in the pseudocolumn
ROWID internally for the construction of indexes.
You can reference rowids in the pseudocolumn
ROWID like other table columns (used in
SELECT lists and
WHERE clauses), but rowids are not stored in the database, nor are they database data. However, you can create tables that contain columns having the
ROWID datatype, although Oracle does not guarantee that the values of such columns are valid rowids. The user must ensure that the data stored in the
ROWID column truly is a valid
See Also:"How Rowids Are Used"
Physical rowids provide the fastest possible access to a row of a given table. They contain the physical address of a row (down to the specific block) and allow you to retrieve the row in a single block access. Oracle guarantees that as long as the row exists, its rowid does not change. These performance and stability qualities make rowids useful for applications that select a set of rows, perform some operations on them, and then access some of the selected rows again, perhaps with the purpose of updating them.
Every row in a nonclustered table is assigned a unique rowid that corresponds to the physical address of a row's row piece (or the initial row piece if the row is chained among multiple row pieces). In the case of clustered tables, rows in different tables that are in the same data block can have the same rowid.
A row's assigned rowid remains unchanged unless the row is exported and imported using the Import and Export utilities. When you delete a row from a table and then commit the encompassing transaction, the deleted row's associated rowid can be assigned to a row inserted in a subsequent transaction.
A physical rowid datatype has one of two formats:
The extended rowid format supports tablespace-relative data block addresses and efficiently identifies rows in partitioned tables and indexes as well as nonpartitioned tables and indexes. Tables and indexes created by an Oracle8i (or higher) server always have extended rowids.
A restricted rowid format is also available for backward compatibility with applications developed with Oracle database version 7 or earlier releases.
SELECT ROWID, last_name FROM employees WHERE department_id = 20;
can return the following row information:
ROWID LAST_NAME ------------------ ---------- AAAAaoAATAAABrXAAA BORTINS AAAAaoAATAAABrXAAE RUGGLES AAAAaoAATAAABrXAAG CHEN AAAAaoAATAAABrXAAN BLUMBERG
OOOOOO: The data object number that identifies the database segment (
AAAAao in the example). Schema objects in the same segment, such as a cluster of tables, have the same data object number.
FFF: The tablespace-relative datafile number of the datafile that contains the row (file
AAT in the example).
BBBBBB: The data block that contains the row (block
AAABrX in the example). Block numbers are relative to their datafile, not tablespace. Therefore, two rows with identical block numbers could reside in two different datafiles of the same tablespace.
RRR: The row in the block.
You can retrieve the data object number from data dictionary views
ALL_OBJECTS. For example, the following query returns the data object number for the
employees table in the
SELECT DATA_OBJECT_ID FROM DBA_OBJECTS WHERE OWNER = 'SCOTT' AND OBJECT_NAME = 'EMPLOYEES';
You can also use the
DBMS_ROWID package to extract information from an extended rowid or to convert a rowid from extended format to restricted format (or vice versa).
See Also:Oracle Database Application Developer's Guide - Fundamentals for information about the
Restricted rowids use a binary representation of the physical address for each row selected. When queried using SQL*Plus, the binary representation is converted to a
VARCHAR2/hexadecimal representation. The following query:
SELECT ROWID, last_name FROM employees WHERE department_id = 30;
can return the following row information:
ROWID ENAME ------------------ ---------- 00000DD5.0000.0001 KRISHNAN 00000DD5.0001.0001 ARBUCKLE 00000DD5.0002.0001 NGUYEN
The data block that contains the row (block DD5 in the example). Block numbers are relative to their datafile, not tablespace. Two rows with identical block numbers could reside in two different datafiles of the same tablespace.
The row in the block that contains the row (rows 0, 1, 2 in the example). Row numbers of a given block always start with 0.
The datafile that contains the row (file 1 in the example). The first datafile of every database is always 1, and file numbers are unique within a database.
You can use the function
SUBSTR to break the data in a rowid into its components. For example, you can use
SUBSTR to break an extended rowid into its four components (database object, file, block, and row):
SELECT ROWID, SUBSTR(ROWID,1,6) "OBJECT", SUBSTR(ROWID,7,3) "FIL", SUBSTR(ROWID,10,6) "BLOCK", SUBSTR(ROWID,16,3) "ROW" FROM products; ROWID OBJECT FIL BLOCK ROW ------------------ ------ --- ------ ---- AAAA8mAALAAAAQkAAA AAAA8m AAL AAAAQk AAA AAAA8mAALAAAAQkAAF AAAA8m AAL AAAAQk AAF AAAA8mAALAAAAQkAAI AAAA8m AAL AAAAQk AAI
Or you can use
SUBSTR to break a restricted rowid into its three components (block, row, and file):
SELECT ROWID, SUBSTR(ROWID,15,4) "FILE", SUBSTR(ROWID,1,8) "BLOCK", SUBSTR(ROWID,10,4) "ROW" FROM products; ROWID FILE BLOCK ROW ------------------ ---- -------- ---- 00000DD5.0000.0001 0001 00000DD5 0000 00000DD5.0001.0001 0001 00000DD5 0001 00000DD5.0002.0001 0001 00000DD5 0002
Rowids can be useful for revealing information about the physical storage of a table's data. For example, if you are interested in the physical location of a table's rows (such as for table striping), the following query of an extended rowid tells how many datafiles contain rows of a given table:
SELECT COUNT(DISTINCT(SUBSTR(ROWID,7,3))) "FILES" FROM tablename; FILES -------- 2
Oracle uses rowids internally for the construction of indexes. Each key in an index is associated with a rowid that points to the associated row's address for fast access. End users and application developers can also use rowids for several important functions:
Rowids are the fastest means of accessing particular rows.
Rowids can be used to see how a table is organized.
Rowids are unique identifiers for rows in a given table.
Before you use rowids in DML statements, they should be verified and guaranteed not to change. The intended rows should be locked so they cannot be deleted. Under some circumstances, requesting data with an invalid rowid could cause a statement to fail.
You can also create tables with columns defined using the
ROWID datatype. For example, you can define an exception table with a column of datatype
ROWID to store the rowids of rows in the database that violate integrity constraints. Columns defined using the
ROWID datatype behave like other table columns: values can be updated, and so on. Each value in a column defined as datatype
ROWID requires six bytes to store pertinent column data.
Rows in index-organized tables do not have permanent physical addresses—they are stored in the index leaves and can move within the block or to a different block as a result of insertions. Therefore their row identifiers cannot be based on physical addresses. Instead, Oracle provides index-organized tables with logical row identifiers, called logical rowids, that are based on the table's primary key. Oracle uses these logical rowids for the construction of secondary indexes on index-organized tables.
Each logical rowid used in a secondary index includes a physical guess, which identifies the block location of the row in the index-organized table at the time the guess was made; that is, when the secondary index was created or rebuilt.
Oracle can use guesses to probe into the leaf block directly, bypassing the full key search. This ensures that rowid access of nonvolatile index-organized tables gives comparable performance to the physical rowid access of ordinary tables. In a volatile table, however, if the guess becomes stale the probe can fail, in which case a primary key search must be performed.
The values of two logical rowids are considered equal if they have the same primary key values but different guesses.
Logical rowids are similar to the physical rowids in the following ways:
Logical rowids are accessible through the
You can use the
ROWID pseudocolumn to select logical rowids from an index-organized table. The
ROWID statement returns an opaque structure, which internally consists of the table's primary key and the physical guess (if any) for the row, along with some control information.
You can access a row using predicates of the form
value is the opaque structure returned by
Access through the logical rowid is the fastest way to get to a specific row, although it can require more than one block access.
A row's logical rowid does not change as long as the primary key value does not change. This is less stable than the physical rowid, which stays immutable through all updates to the row.
Logical rowids can be stored in a column of the
One difference between physical and logical rowids is that logical rowids cannot be used to see how a table is organized.
Note:An opaque type is one whose internal structure is not known to the database. The database provides storage for the type. The type designer can provide access to the contents of the type by implementing functions, typically 3GL routines.
See Also:"Overview of ROWID and UROWID Datatypes"
When a row's physical location changes, the logical rowid remains valid even if it contains a guess, although the guess could become stale and slow down access to the row. Guess information cannot be updated dynamically. For secondary indexes on index-organized tables, however, you can rebuild the index to obtain fresh guesses. Note that rebuilding a secondary index on an index-organized table involves reading the base table, unlike rebuilding an index on an ordinary table.
Collect index statistics with the
DBMS_STATS package or
ANALYZE statement to keep track of the staleness of guesses, so Oracle does not use them unnecessarily. This is particularly important for applications that store rowids with guesses persistently in a
UROWID column, then retrieve the rowids later and use them to fetch rows.
When you collect index statistics with the
DBMS_STATS package or
ANALYZE statement, Oracle checks whether the existing guesses are still valid and records the percentage of stale/valid guesses in the data dictionary. After you rebuild a secondary index (recomputing the guesses), collect index statistics again.
In general, logical rowids without guesses provide the fastest possible access for a highly volatile table. If a table is static or if the time between getting a rowid and using it is sufficiently short to make row movement unlikely, logical rowids with guesses provide the fastest access.
See Also:Oracle Database Performance Tuning Guide for more information about collecting statistics
Oracle database applications can be run against non-Oracle database servers using SQL*Connect. The format of rowids varies according to the characteristics of the non-Oracle system. Furthermore, no standard translation to
VARCHAR2/hexadecimal format is available. Programs can still use the
ROWID datatype. However, they must use a nonstandard translation to hexadecimal format of length up to 256 bytes.
Rowids of a non-Oracle database can be stored in a column of the
SQL statements that create tables and clusters can also use ANSI datatypes and datatypes from IBM's products SQL/DS and DB2. Oracle recognizes the ANSI or IBM datatype name that differs from the Oracle datatype name, records it as the name of the datatype of the column, and then stores the column's data in an Oracle datatype based on the conversions.
See Also:Oracle Database SQL Reference for more information about the conversions
Oracle provides the
XMLType datatype to handle XML data.
XMLType can be used like any other user-defined type.
XMLType can be used as the datatype of columns in tables and views. Variables of
XMLType can be used in PL/SQL stored procedures as parameters, return values, and so on. You can also use XMLType in PL/SQL, SQL and Java, and through JDBC and OCI.
A number of useful functions that operate on XML content have been provided. Many of these are provided both as SQL functions and as member functions of
XMLType. For example, function
extract extracts a specific node(s) from an
XMLType instance. You can use
XMLType in SQL queries in the same way as any other user-defined datatypes in the system.
Oracle Streams Advanced Queuing User's Guide and Reference for information about using
XMLType with Advanced Queuing
A URI, or uniform resource identifier, is a generalized kind of URL. Like a URL, it can reference any document, and can reference a specific part of a document. It is more general than a URL because it has a powerful mechanism for specifying the relevant part of the document. By using
UriType, you can do the following:
Create table columns that point to data inside or outside the database.
Query the database columns using functions provided by
See Also:Oracle XML DB Developer's Guide
See Also:Oracle Database SQL Reference for the rules for implicit datatype conversions