Glosario de Bases de Datos Sybase, "I,J,K,L"

identifier

A string of characters used to identify a database object, such as a table name or column name.

IDENTITY column

A column that contains system-generated values that uniquely identify each row in a table. IDENTITY columns store unique numbers, such as invoice numbers or employee numbers, that are generated automatically by Adaptive Server. The value of the IDENTITY column uniquely identifies each row in a table.

image function

See text and image function .

implicit conversions

Datatype conversions that Adaptive Server automatically performs to compare datatypes.

index

A database object that consists of key values from the data tables and pointers to the pages that contain those values. Indexes speed up access to data rows by pointing Adaptive Server to the location of a table column's data on disk.

index covering

A data access condition where the leaf-level pages of a nonclustered index contain the data needed to satisfy a query. The index must contain all columns in the select list as well as the columns in the query clauses, if any. The server can satisfy the query using only the leaf level of the index. When an index covers a query, the server does not access the data pages.

index selectivity

The ratio of duplicate key values in an index. An index is selective when it lets the optimizer pinpoint a single row, such as a search for a unique key. An index on nonunique entries is less selective. An index on values such as "M" or "F" (for male or female) is extremely nonselective.

initial response time

The time required to return the first result row of a query to a user. For some queries, initial response time can be very brief, even though time to return the full result set can take much longer.

initialization

See disk initialization .

inner query

See subquery .

in-place update

A type of direct update operation. An in-place update does not cause data rows to move on the data page. Compare to on-page update and delete/insert direct update .

int

A signed 32-bit integer value.

integrity constraints

A model to describe the database integrity in the create table statement.

integrity rules

Rules that describe how data will be kept accurate and consistent in the relational model. See also database integrity .

intent lock

A type of table-level lock. A shared intent lock indicates that there is a shared lock on a page of that table. It prevents other transactions from acquiring an exclusive table-level lock. An exclusive intent lock indicates that there is an exclusive or update lock on a page of that table. It prevents other transactions from acquiring an exclusive or shared table-level lock.

I/O

See device I/O .

isolation level

Specifies the kinds of actions that are not permitted while the current transactions execute; also called "locking level." The ANSI standard defines four levels of isolation for SQL transactions. Level 0 prevents other transactions from changing data already modified by an uncommitted transaction. Level 1 prevents dirty reads. Level 2 (not supported by Adaptive Server) also prevents non-repeatable reads . Level 3 prevents both types of reads and phantoms; it is equivalent to doing all querieswith holdlock. The user controls the isolation level with the set option transaction isolation level or with the at isolation clause of select or readtext . The default is level 1.

join

A basic operation in a relational system that links the rows in two or more tables by comparing the values in specified columns.

join selectivity

An estimate of the number of rows from a particular table that will join with a row from another table. If index statistics are available for the join column, Adaptive Server bases the join selectivity on the density of the index (the average number of duplicate rows). If no statistics are available, the selectivity is 1/ N, where N is the number of rows in the smaller table.

kernel

A module within Adaptive Server that acts as the interface between Adaptive Server and the operating system. The kernel manages tasks associated with Adaptive Server's clients. For example, it tracks the state of the task, which is necessary in a multithreaded environment because of context switching.

key

A field used to identify a record, often used as the index field for a table.

key value

Any value that is indexed.

keyword

A word or phrase that is reserved for exclusive use by Transact-SQL. Also known as a reserved word.

keyword phrase

See clause .

language cursor

A cursor declared in SQL without using Open Client. As with Adaptive Server cursors, Open Client is completely unaware of the cursors and the results are sent back to the client in the same format as a normal select.

Language Module

A set of files, including localization files, that provide alternate language, sort order, and character sets for Adaptive Server.

last-chance threshold

A default threshold in Adaptive Server that suspends or kills user processes if the transaction log has run out of room. This threshold leaves just enough space for the de-allocation records for the log itself. The last-chance threshold always calls a procedure named sp_thresholdaction. This procedure is not supplied by Sybase. It must be written by the System Administrator.

leaf level

The level of an index at which all key values appear in order. For Adaptive Server clustered indexes, the leaf level and the data level are the same. For nonclustered indexes, the last index level above the data level is the leaf level, since key values for all of the data rows appear there in sorted order.

legacy data

Data from older, perhaps out-of-date, data sources or from data sources that are no longer supported by current standards.

libraries

See Application Program Interface (API) .

lightweight process

See worker process .

livelock

A request for an exclusive lock that is repeatedly denied because a series of overlapping shared locks keeps interfering. Adaptive Server detects the situation after three denials and grants a demand lock to the update transaction, queueing further shared lock requests after the demand lock.

load

To restore data stored in a backup created during a dump .

localization files

Files that contain translated error messages, character set and sort order definitions, and utilities for converting Adaptive Server's character set into the appropriate character set for a particular terminal.

local variable

A user-defined variable defined with a declare statement. See also global variable .

location transparency

A feature unique to Component Integration Services that makes remote data appear as if it were local. Users do not need to know where the data resides in order to access it.

lock

A concurrency control mechanism that protects the integrity of data and transaction results in a multiuser environment. Adaptive Server applies page or table locks to prevent two users from attempting to change the same data at the same time, and to prevent processes that are selecting data from reading data that is in the process of being changed.

lock promotion threshold

The number of page locks allowed in a table before Adaptive Server attempts to issue a table lock. If the table lock is successful, Adaptive Server releases the page locks.

locking

The process of restricting access to resources in a multi-user environment to maintain security and prevent concurrent access problems. Adaptive Server automatically applies locks to tables or pages.

locking level

See isolation level.

logical design

A design in which you define the tables, relations, and keys of a relational database. See also physical design .

logical expression

An expression that evaluates to TRUE (1), FALSE (0) or UNKNOWN (NULL). Logical expressions are often used in control of flow statements, such as if or while conditions.

logical key

The primary, foreign, or common key definitions in a database design that define the relationship between tables in the database. Logical keys are not necessarily the same as the physical keys (the keys used to create indexes) on the table.

logical operators

The operators and, or, and not. All three can be used in where clauses. The operator and joins two or more conditions and returns results when all of the conditions are true; or connects two or more conditions and returns results when any of the conditions is true.

logical read

The process of accessing a data or index page already in memory to satisfy a query. See also physical read.

login

The name a user uses to log into Adaptive Server. A login is valid if Adaptive Server has an entry for that user in the system table syslogins .

look-ahead set

The set of pages for a particular operation to be fetched by asynchronous prefetch. Each Adaptive Server operation that uses asynchronous prefetch builds a look-ahead set based on the known or expected set of pages that will be needed by the operation in the near future.

LRU buffer reuse strategy

A caching strategy for replacing the least-recently used buffers in the data cache. A clean data page is taken from the LRU end of the data cache to store a page read from disk. The new page is placed on the data cache's page chain at the MRU end of the cache, so that it stays in memory.