Class BTreeMaxScan

java.lang.Object

org.apache.derby.impl.store.access.btree.OpenBTree

org.apache.derby.impl.store.access.btree.BTreeScan

org.apache.derby.impl.store.access.btree.BTreeMaxScan

All Implemented Interfaces:

ScanManager, GenericScanController, GroupFetchScanController, RowCountable, ScanController

Direct Known Subclasses:

B2IMaxScan

public class BTreeMaxScan
extends BTreeScan

A BTreeScan implementation that provides the 95% solution to the max on btree problem. If the row is the last row in the btree it works very efficiently. This implementation will be removed once backward scan is fully functional. The current implementation only exports to the user the ability to call fetchMax() and get back one row, none of the generic scan ablities are exported. To return the maximum row this implementation does the following: 1) calls positionAtStartPosition() which returns with the a latch on the rightmost leaf page and a lock on the rightmost leaf row on that page. It will loop until it can get the lock without waiting while holding the latch. At this point the slot position is just right of the locked row. 2) in fetchMax() it loops backward on the last leaf page, locking rows as it does so, until it finds the first non-deleted, non-NULL row. 3) If it is not successful in this last page search it attempts to latch the left sibling page, without waiting to avoid deadlocks with forward scans, and continue the search on that page. 4) If the sibling page couldn't be latched without waiting, save the current position, release all latches, and restart the scan from the saved position.

Field Summary

Fields inherited from class BTreeScan

fetchNext_one_slot_array, init_fetchDesc, init_forUpdate, init_hold, init_lock_fetch_desc, init_qualifier, init_rawtran, init_scanColumnList, init_startKeyValue, init_startSearchOperator, init_stopKeyValue, init_stopSearchOperator, init_template, init_useUpdateLocks, lock_operation, SCAN_DONE, SCAN_HOLD_INIT, SCAN_HOLD_INPROGRESS, SCAN_INIT, SCAN_INPROGRESS, scan_position, scan_state, stat_numdeleted_rows_visited, stat_numpages_visited, stat_numrows_qualified, stat_numrows_visited

Fields inherited from class OpenBTree

btree_undo, container, err_containerid, init_lock_level, init_open_user_scans, runtime_mem

Fields inherited from interface ScanController

GE, GT, NA

Constructor Summary

Constructors

Constructor	Description
BTreeMaxScan()

Method Summary

Modifier and Type	Method	Description
boolean	fetchMax(DataValueDescriptor[] fetch_row)	Fetch the maximum row in the table.
protected int	fetchRows(BTreeRowPosition pos, DataValueDescriptor[][] row_array, RowLocation[] rowloc_array, BackingStoreHashtable hash_table, long max_rowcnt, int[] key_column_numbers)	disallow fetchRows on this scan type, caller should only be able to call fetchMax().
private boolean	moveToLeftSibling()	Move the current position to the page to the left of the current page, right after the last slot on that page.
protected void	positionAtStartPosition(BTreeRowPosition pos)	Position scan at "start" position of the MAX scan.

Methods inherited from class BTreeScan

close, closeForEndTransaction, delete, didNotQualify, doesCurrentPositionQualify, fetch, fetchLocation, fetchNext, fetchNextGroup, fetchNextGroup, fetchSet, fetchWithoutQualify, getCurrentRecordHandleForDebugging, getScanInfo, init, isCurrentPositionDeleted, isEmpty, isHeldAfterCommit, isKeyed, newRowLocationTemplate, next, positionAtDoneScan, positionAtDoneScanFromClose, positionAtNextPage, positionAtPreviousPage, positionAtRowLocation, positionAtStartForForwardScan, process_qualifier, reopenScan, reopenScanByRowLocation, replace, reposition, savePositionAndReleasePage, savePositionAndReleasePage, toString

Methods inherited from class OpenBTree

checkConsistency, debugConglomerate, getColumnSortOrderInfo, getConglomerate, getContainer, getContainerHandle, getEstimatedRowCount, getHeight, getHold, getLockingPolicy, getLockLevel, getOpenMode, getRawTran, getRuntimeMem, getSpaceInfo, getXactMgr, init, isClosed, isIndexableRowConsistent, isTableLocked, reopen, setEstimatedRowCount, setLockingPolicy, test_errors

Methods inherited from class Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface GenericScanController

isTableLocked

Methods inherited from interface RowCountable

getEstimatedRowCount, setEstimatedRowCount

Constructor Details

BTreeMaxScan

public BTreeMaxScan()

Method Details

moveToLeftSibling

private boolean moveToLeftSibling()
                           throws StandardException

Move the current position to the page to the left of the current page, right after the last slot on that page. If we have to wait for a latch, give up the latch on the current leaf and give up. The caller will have to reposition and retry.

Returns:

true if the position was successfully moved, false if we had to wait for a latch

Throws:

StandardException

fetchRows

protected int fetchRows(BTreeRowPosition pos,
 DataValueDescriptor[][] row_array,
 RowLocation[] rowloc_array,
 BackingStoreHashtable hash_table,
 long max_rowcnt,
 int[] key_column_numbers)
                 throws StandardException

disallow fetchRows on this scan type, caller should only be able to call fetchMax().

Specified by:

fetchRows in class BTreeScan

Throws:

StandardException - Standard exception policy.

positionAtStartPosition

protected void positionAtStartPosition(BTreeRowPosition pos)
                                throws StandardException

Position scan at "start" position of the MAX scan.

Positions the scan to the slot just after the last record on the rightmost leaf of the index. Returns the rightmost leaf page latched, the rightmost row on the page locked and sets "current_slot" to the slot number just right of the last row on the page.

Specified by:

positionAtStartPosition in class BTreeScan

Throws:

StandardException - Standard exception policy.

fetchMax

public boolean fetchMax(DataValueDescriptor[] fetch_row)
                 throws StandardException

Fetch the maximum row in the table. Call positionAtStartPosition() to quickly position on rightmost row of rightmost leaf of tree. Search last page for last non deleted row, and if one is found return it as max. If no row found on last page, or could not find row without losing latch then call fetchMaxRowFromBeginning() to search from left to right for maximum value in index.

Throws:

StandardException - Standard exception policy.