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org.apache.derby.iapi.store.access
Interface GroupFetchScanController  

All Superinterfaces:

GenericScanController, RowCountable

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public interface GroupFetchScanController

extends GenericScanController

This scan controller can only be used for group fetch, no update operations are supported, use ScanController if you need scan interfaces other than group fetch.

In general group fetch will be more efficient than using the ScanController fetchNext() interface to get rows one at a time. The performance comes from reducing the per call overhead of getting a row. Also this interface can, depending on the requested isolation level, possibly do more efficient locking.

Group fetch scans are opened from a TransactionController.

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Method Summary
int	fetchNextGroup(org.apache.derby.iapi.types.DataValueDescriptor[][] row_array, org.apache.derby.iapi.types.RowLocation[] rowloc_array)           Fetch the next N rows from the table.
int	fetchNextGroup(org.apache.derby.iapi.types.DataValueDescriptor[][] row_array, org.apache.derby.iapi.types.RowLocation[] oldrowloc_array, org.apache.derby.iapi.types.RowLocation[] newrowloc_array)
boolean	next()           Move to the next position in the scan.

 

Methods inherited from interface org.apache.derby.iapi.store.access.GenericScanController

close, getScanInfo, isKeyed, isTableLocked, newRowLocationTemplate, reopenScan, reopenScanByRowLocation

 

Methods inherited from interface org.apache.derby.iapi.store.access.RowCountable

getEstimatedRowCount, setEstimatedRowCount

 

Method Detail

fetchNextGroup

public int fetchNextGroup(org.apache.derby.iapi.types.DataValueDescriptor[][] row_array,
                          org.apache.derby.iapi.types.RowLocation[] rowloc_array)
                   throws org.apache.derby.iapi.error.StandardException

Fetch the next N rows from the table.

The client allocates an array of N rows and passes it into the fetchNextSet() call. The client must at least allocate a row and set row_array[0] to this row. The client can optionally either leave the rest of array entries null, or allocate rows to the slots. If access finds an entry to be null, and wants to read a row into it, it will allocate a row to the slot. Once fetchNextGroup() returns "ownership" of the row passes back to the client, access will not keep references to the allocated row. Expected usage is that the client will specify an array of some number (say 10), and then only allocate a single row. This way if only 1 row qualifies only one row will have been allocated.

This routine does the equivalent of N fetchNext() calls, filling in each of the rows in the array. Locking is performed exactly as if the N fetchNext() calls had been made.

It is up to Access how many rows to return. fetchNextGroup() will return how many rows were filled in. If fetchNextGroup() returns 0 then the scan is complete, (ie. the scan is in the same state as if fetchNext() had returned false). If the scan is not complete then fetchNext() will return (1 <= row_count <= N).

The current position of the scan is undefined if fetchNextSet() is used (ie. mixing fetch()/fetchNext() and fetchNextSet() calls in a single scan does not work). This is because a fetchNextSet() request for 5 rows from a heap where the first 2 rows qualify, but no other rows qualify will result in the scan being positioned at the end of the table, while if 5 rows did qualify the scan will be positioned on the 5th row.

If the row loc array is non-null then for each row fetched into the row array, a corresponding fetchLocation() call will be made to fill in the rowloc_array. This array, like the row array can be initialized with only one non-null RowLocation and access will allocate the rest on demand.

Qualifiers, start and stop positioning of the openscan are applied just as in a normal scan.

The columns of the row will be the standard columns returned as part of a scan, as described by the validColumns - see openScan for description.

Expected usage: // allocate an array of 5 empty rows DataValueDescriptor[][] row_array = allocate_row_array(5); int row_cnt = 0; scan = openScan(); while ((row_cnt = scan.fetchNextSet(row_array, null) != 0) { // I got "row_cnt" rows from the scan. These rows will be // found in row_array[0] through row_array[row_cnt - 1] }

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fetchNextGroup

public int fetchNextGroup(org.apache.derby.iapi.types.DataValueDescriptor[][] row_array,
                          org.apache.derby.iapi.types.RowLocation[] oldrowloc_array,
                          org.apache.derby.iapi.types.RowLocation[] newrowloc_array)
                   throws org.apache.derby.iapi.error.StandardException

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next

public boolean next()
             throws org.apache.derby.iapi.error.StandardException

Move to the next position in the scan. If this is the first call to next(), the position is set to the first row. Returns false if there is not a next row to move to. It is possible, but not guaranteed, that this method could return true again, after returning false, if some other operation in the same transaction appended a row to the underlying conglomerate.

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Overview  Package   Class  Use  Deprecated  Index
Home >> All >> org >> apache >> derby >> iapi >> store >> [ access overview ]	PREV CLASS  NEXT CLASS
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