JetMove Function

Artikkel
07/13/2022

Applies to: Windows | Windows Server

JetMove Function

The JetMove function positions a cursor at the start or end of an index and traverses the entries in that index either forward or backward. It is also possible to move the cursor forward or backward on the current index by a specified number of index entries. Another approach is to artificially limit the index entries that can be enumerated using JetMove by setting up an index range on the cursor using JetSetIndexRange.

    JET_ERR JET_API JetMove(
      __in          JET_SESID sesid,
      __in          JET_TABLEID tableid,
      __in          long cRow,
      __in          JET_GRBIT grbit
    );

Parameters

sesid

The session to use for this call.

tableid

The cursor to use for this call.

cRow

An arbitrary offset that indicates the desired movement of the cursor on the current index.

In addition to standard offsets, this parameter can also be set with one of the following options.

Value	Meaning
JET_MoveFirst	Moves the cursor to the first index entry in the index (if one exists). This resets any index range set with JetSetIndexRange. Note The literal value of -2147483648 is used to denote this option. Do not use this value as an ordinary offset or unintended behavior may result.
JET_MoveLast	Moves the cursor to the last index entry in the index (if one exists). This resets any index range set with JetSetIndexRange. Note The literal value of 2147483647 is used to denote this option. Do not use this value as an ordinary offset or unintended behavior may result.
JET_MoveNext	Moves the cursor to the next index entry in the index (if one exists). This value is exactly equal to an ordinary offset of +1. It respects index ranges set with JetSetIndexRange.
JET_MovePrevious	Moves the cursor to the previous index entry in the index (if one exists). It respects index ranges set with JetSetIndexRange. This value is exactly equal to an ordinary offset of -1, or 0 (Zero). The cursor remains at the current logical position and the existence of the index entry that corresponds to that logical position will be tested.

grbit

A group of bits that specify zero or more of the following options.

Value	Meaning
JET_bitMoveKeyNE	Moves the cursor forward or backward by the number of index entries required to skip the requested number of index key values encountered in the index. This has the effect of collapsing index entries with duplicate key values into a single index entry. Ordinarily, an offset will move the cursor by the specified number of index entries regardless of their key values.

Return Value

This function returns the JET_ERR datatype with one of the following return codes. For more information about the possible ESE errors, see Extensible Storage Engine Errors and Error Handling Parameters.

Return code	Description
JET_errSuccess	The operation completed successfully. For JetMove, this means that an index entry was found at the requested location or offset on the current index.
JET_errClientRequestToStopJetService	The operation cannot complete because all activity on the instance that is associated with the session has ceased as a result of a call to JetStopService.
JET_errInstanceUnavailable	The operation cannot complete because the instance that is associated with the session encountered a fatal error that requires that access to all data be revoked to protect the integrity of that data. Windows XP: This return value is introduced in Windows XP.
JET_errNoCurrentRecord	The cursor is not currently positioned on an index entry. For JetMove, this means that an index entry was not found at the requested location or offset on the current index.
JET_errNotInitialized	The operation cannot complete because the instance that is associated with the session has not yet been initialized.
JET_errRecordDeleted	The cursor is currently logically positioned on an index entry that corresponds to a record that has been deleted.
JET_errRestoreInProgress	The operation cannot complete because a restore operation is in progress on the instance associated with the session.
JET_errSessionSharingViolation	The same session cannot be used for more than one thread at the same time. Windows XP: This return value is introduced in Windows XP.
JET_errTermInProgress	The operation cannot complete because the instance that is associated with the session is being shut down.

If this function succeeds, the cursor will be positioned at an index entry that matches the requested location or offset. If a record has been prepared for update, that update will be canceled. If an index range is in effect and JET_MoveFirst or JET_MoveLast was specified, that index range will be canceled. No change to the database state will occur.

If this function fails, the position of the cursor will remain unchanged unless JET_errNoCurrentRecord was returned. In that case, the cursor will be positioned where the index entry that matches the requested location or offset would have been. The cursor can be moved relative to that position but is still not on a valid index entry. If a record has been prepared for update, that update will be canceled. If an index range is in effect and JET_MoveFirst or JET_MoveLast was specified, that index range will be canceled. No change to the database state will occur.

Remarks

A cursor can be moved to two special positions using JetMove, Before First and After Last. If the cursor is on the first index entry in the table and JetMove is called with JET_MovePrevious, the call will fail with JET_errNoCurrentRecord and the cursor will be logically positioned before the first entry in the index. This logical position is maintained even if another index entry is inserted prior to the first entry in the index. An analogous situation occurs for After Last relative to the end of the index. Before First and After Last are most useful when setting up a range of index entries to be iterated using JetMove, using an iterator model that expects to always move to the next (or previous) element prior to using that element.

The set of index entries that can be visited by JetMove can be limited by setting up an index range on the cursor. This is useful for applications that enumerate a set of index entries that match simple criteria that can be expressed through a pair of search keys built for that index. For more information, see JetSetIndexRange.

On releases prior to Windows Server 2003 SP1, there is a problem in JetMove that occurs in some specific cases, that affects the correct enforcement of an index range as set up by the JetSetIndexRange function. If the cursor is currently before the first index entry and an index range is set with an upper limit that is less than the first index entry, the next call to JetMove will erroneously go to that index entry instead of failing with JET_errNoCurrentRecord, as expected. The same error will happen for the analogous situation starting from the end of the index. This situation can occur in an application that sets up an index range and navigates it using an iterator that expects to start before the first index entry that is a member of the set of entries to enumerate, rather than starting on the first index entry of that set. This situation also occurs on the analogous case starting from the end of the index. The workaround is for the application to manually verify that the index entry acquired is inside the index range by comparing the key for the current index entry (retrieved using JetRetrieveKey) with the key representing the end of the current index range (retrieved using JetRetrieveKey using JET_bitRetrieveCopy).

It is important to be careful when passing computed offsets to JetMove. If the computed offset is less than or equal to JET_MoveFirst, that offset must be broken up into multiple pieces, each of which is passed to JetMove separately but within a single transaction to get the desired effect. The same is true for offsets greater than or equal to JET_MoveNext. It is unlikely that an application would undergo this in the real world, but it is good to have a defense against this case given the very different semantics of JET_MoveFirst and JET_MoveLast from an ordinary offset.

When JetMove is called with a very large offset, such as when the cRow parameter is set to 1000, JetMove traverses all 1000 index entries to reach the final position. Currently, the ESE API does not provide an efficient way to move directly to a given index entry by offset without traversing each index entry.

Requirements

Requirement	Value
Client	Requires Windows Vista, Windows XP, or Windows 2000 Professional.
Server	Requires Windows Server 2008, Windows Server 2003, or Windows 2000 Server.
Header	Declared in Esent.h.
Library	Use ESENT.lib.
DLL	Requires ESENT.dll.

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JetMove Function