Class GenericScanController

  • All Implemented Interfaces:
    ScanManager, GenericScanController, GroupFetchScanController, RowCountable, ScanController
    Direct Known Subclasses:
    HeapScan

    public abstract class GenericScanController
    extends GenericController
    implements ScanManager
    Generic class implementing shared ScanController methods. Logically a scancontroller is used to scan a set of rows that meet some specified qualification. Rows that meet the qualification may be operated upon by the scan to fetch, delete, or replace. The ScanController also supports the notion or "repositioning" the scan, which simply resets the beginning of the scan to a new place, and allows the user to continue from there. This class attempts to abstract out some of the parts of the scan such that maybe multiple access methods can share code, even if they perform parts of the scan wildly differently. Here is how the scan has been broken apart: scan_position - this variable holds the current scan position, it may be extended to provide more information if necessary. scan_state - a scan has 5 possible states: SCAN_INIT, SCAN_INPROGRESS, SCAN_DONE, SCAN_HOLD_INIT, and SCAN_HOLD_INPROGRESS positionAtInitScan() - This routine is called to move the scan to the SCAN_INIT state. It is used both for initialization of the ScanController and by reopenScan(). positionAtStartForForwardScan() - This routine is called to move the scan from SCAN_INIT to SCAN_INPROGRESS. Upon return from this routine it is expected that scan_position is set such that calling the generic scan loop will reach the first row of the scan. Note that this usually means setting the scan_postion to one before the 1st row to be returned. fetchRows() - This routine is the meat of the scan, it moves the scan to the next row, applies necessary qualifiers, and handles group or non-group operations. It moves through rows on a page in order and then moves to the "next" page. positionAtNextPage() - This routine handles moving the scan from the current scan_position to the next page. positionAtDoneScan() - Handle all cleanup associated with moving the scan state from SCAN_INPROGRESS to SCAN_DONE. This may include releasing locks, and setting the state of the scan. This does not close the scan, it allows for a reopenScan() to be called.
    • Field Detail

      • init_scanColumnList

        private FormatableBitSet init_scanColumnList
        The following group of fields are all basic input parameters which are provided by the calling code when doing a scan. These are just saved values from what was initially input.
      • init_startSearchOperator

        private int init_startSearchOperator
      • init_qualifier

        private Qualifier[][] init_qualifier
      • init_stopSearchOperator

        private int init_stopSearchOperator
      • scan_state

        private int scan_state
        Delay positioning the table at the start position until the first next() call.
      • rowLocationsInvalidated

        protected boolean rowLocationsInvalidated
        If this flag is set to true, a RowLocation returned from this controller may have been reused for another row.
      • reusableRecordIdSequenceNumber

        private long reusableRecordIdSequenceNumber
        This is the sequence number for when a record id can be reused. If it has been changed in the container, a RowLocation may be reused for another row.
      • scan_position

        protected RowPosition scan_position
        The position for the current scan. The can be maintained in any of the following ways: record handle - scan_position.current_rh: The scan maintains it's position using the record handle while it does not have a latch on the page, which is the case anytime control leaves access. The access method must take appropriate steps to make sure the record handle will still be valid when the scan needs to reposition using the record handle. slot number - scan_position.current_slot: While the scan has a latch on the page the scan is positioned using the slot number as the order of the rows cannot change while the latch is held (unless the holder of the latch causes them to move). page number - (RESOLVE - TODO) Sometimes it would be interesting to position a scan "between" pages, such that the next time the scan starts is starts at the next page. This would allow us to efficiently do group scans returning page at atime results. NOT IMPLEMENTED CURRENTLY.
      • stat_numpages_visited

        protected int stat_numpages_visited
        Performance counters ...
      • stat_numrows_visited

        protected int stat_numrows_visited
      • stat_numrows_qualified

        protected int stat_numrows_qualified
    • Constructor Detail

      • GenericScanController

        public GenericScanController()
    • Method Detail

      • positionAtInitScan

        protected void positionAtInitScan​(DataValueDescriptor[] startKeyValue,
                                          int startSearchOperator,
                                          Qualifier[][] qualifier,
                                          DataValueDescriptor[] stopKeyValue,
                                          int stopSearchOperator,
                                          RowPosition pos)
                                   throws StandardException
        Move scan to the the SCAN_INIT state.

        This routine is called to move the scan to the SCAN_INIT state. It is used both for initialization of the ScanController and by reopenScan().

        Throws:
        StandardException
      • positionAtResumeScan

        protected void positionAtResumeScan​(RowPosition pos)
                                     throws StandardException
        Reposition the scan upon entering the fetchRows loop.

        Called upon entering fetchRows() while in the SCAN_INPROGRESS state. Do work necessary to look at rows in the current page of the scan.

        The default implementation uses a record handle to maintain a scan position. It will get the latch again on the current scan position and set the slot to the current record handle.

        Throws:
        StandardException - Standard exception policy.
      • positionAtStartForForwardScan

        protected void positionAtStartForForwardScan​(RowPosition pos)
                                              throws StandardException
        Move the scan from SCAN_INIT to SCAN_INPROGRESS.

        This routine is called to move the scan from SCAN_INIT to SCAN_INPROGRESS. Upon return from this routine it is expected that scan_position is set such that calling the generic scan loop will reach the first row of the scan. Note that this usually means setting the scan_postion to one before the 1st row to be returned.

        Throws:
        StandardException - Standard exception policy.
      • reopenScanByRowLocation

        public void reopenScanByRowLocation​(RowLocation startRowLocation,
                                            Qualifier[][] qualifier)
                                     throws StandardException
        Description copied from interface: GenericScanController
        Reposition the current scan. This call is semantically the same as if the current scan had been closed and a openScan() had been called instead. The scan is reopened against the same conglomerate, and the scan is reopened with the same "scan column list", "hold" and "forUpdate" parameters passed in the original openScan.

        The statistics gathered by the scan are not reset to 0 by a reopenScan(), rather they continue to accumulate.

        Note that this operation is currently only supported on Heap conglomerates. Also note that order of rows within are heap are not guaranteed, so for instance positioning at a RowLocation in the "middle" of a heap, then inserting more data, then continuing the scan is not guaranteed to see the new rows - they may be put in the "beginning" of the heap.

        Specified by:
        reopenScanByRowLocation in interface GenericScanController
        Parameters:
        startRowLocation - An existing RowLocation within the conglomerate, at which to position the start of the scan. The scan will begin at this location and continue forward until the end of the conglomerate. Positioning at a non-existent RowLocation (ie. an invalid one or one that had been deleted), will result in an exception being thrown when the first next operation is attempted.
        qualifier - An array of qualifiers which, applied to each key, restrict the rows returned by the scan. Rows for which any one of the qualifiers returns false are not returned by the scan. If null, all rows are returned.
        Throws:
        StandardException - Standard exception policy.
      • allocateScanPosition

        protected RowPosition allocateScanPosition()
                                            throws StandardException
        Create object which represents the scan position.

        Designed so that extending classes can override and allocate implementation specific row position's.

        Throws:
        StandardException - Standard exception policy.
      • reopenScanByRecordHandle

        protected void reopenScanByRecordHandle​(RecordHandle startRecordHandle,
                                                Qualifier[][] qualifier)
                                         throws StandardException
        Reposition the current scan. This call is semantically the same as if the current scan had been closed and a openScan() had been called instead. The scan is reopened against the same conglomerate, and the scan is reopened with the same "scan column list", "hold" and "forUpdate" parameters passed in the original openScan.

        The statistics gathered by the scan are not reset to 0 by a reopenScan(), rather they continue to accumulate.

        Note that this operation is currently only supported on Heap conglomerates. Also note that order of rows within are heap are not guaranteed, so for instance positioning at a RowLocation in the "middle" of a heap, then inserting more data, then continuing the scan is not guaranteed to see the new rows - they may be put in the "beginning" of the heap.

        Parameters:
        startRecordHandle - An existing RecordHandle within the conglomerate, at which to position the start of the scan. The scan will begin at this location and continue forward until the end of the conglomerate. Positioning at a non-existent RowLocation (ie. an invalid one or one that had been deleted), will result in an exception being thrown when the first next operation is attempted.
        qualifier - An array of qualifiers which, applied to each key, restrict the rows returned by the scan. Rows for which any one of the qualifiers returns false are not returned by the scan. If null, all rows are returned.
        Throws:
        StandardException - Standard exception policy.
      • getNumPagesVisited

        public final int getNumPagesVisited()
      • getNumRowsVisited

        public final int getNumRowsVisited()
      • getNumRowsQualified

        public final int getNumRowsQualified()
      • getStartSearchOperator

        public final int getStartSearchOperator()
      • getStopSearchOperator

        public final int getStopSearchOperator()
      • getQualifier

        public final Qualifier[][] getQualifier()
      • getScanState

        public final int getScanState()
      • setScanState

        public final void setScanState​(int state)
      • getScanPosition

        public final RowPosition getScanPosition()
      • setScanPosition

        public final void setScanPosition​(RowPosition pos)
      • reopenAfterEndTransaction

        protected final boolean reopenAfterEndTransaction()
                                                   throws StandardException
        Reopens the scan after it has been closed as part of a commit. This method will check the reusableRecordIdSequenceNumber of the container, and will set the rowLocationsInvalidated flag if it has changed.
        Returns:
        true if the conglomerate has been reopened
        Throws:
        StandardException - Derby standard exception
      • closeForEndTransaction

        public boolean closeForEndTransaction​(boolean closeHeldScan)
                                       throws StandardException
        Description copied from interface: ScanManager
        Close scan as part of terminating a transaction.

        Use this call to close the scan resources as part of committing or aborting a transaction. The normal close() routine may do some cleanup that is either unnecessary, or not correct due to the unknown condition of the scan following a transaction ending error. Use this call when closing all scans as part of an abort of a transaction.

        Specified by:
        closeForEndTransaction in interface ScanManager
        Parameters:
        closeHeldScan - If true, means to close scan even if it has been opened to be kept opened across commit. This is used to close these scans on abort.
        Returns:
        boolean indicating that the close has resulted in a real close of the scan. A held scan will return false if called by closeForEndTransaction(false), otherwise it will return true. A non-held scan will always return true.
        Throws:
        StandardException - Standard exception policy.
      • didNotQualify

        public void didNotQualify()
                           throws StandardException
        A call to allow client to indicate that current row does not qualify.

        Indicates to the ScanController that the current row does not qualify for the scan. If the isolation level of the scan allows, this may result in the scan releasing the lock on this row.

        Note that some scan implimentations may not support releasing locks on non-qualifying rows, or may delay releasing the lock until sometime later in the scan (ie. it may be necessary to keep the lock until either the scan is repositioned on the next row or page).

        This call should only be made while the scan is positioned on a current valid row. RESOLVE (mikem-05/29/98) - Implement this when we support levels of concurrency less than serializable.

        Specified by:
        didNotQualify in interface ScanController
        Throws:
        StandardException - Standard exception policy.
      • fetchSet

        public void fetchSet​(long max_rowcnt,
                             int[] key_column_numbers,
                             BackingStoreHashtable hash_table)
                      throws StandardException
        Insert all rows that qualify for the current scan into the input Hash table.

        This routine scans executes the entire scan as described in the openScan call. For every qualifying unique row value an entry is placed into the HashTable. For unique row values the entry in the Hashtable has a key value of the object stored in row[key_column_number], and the value of the data is row. For row values with duplicates, the key value is also row[key_column_number], but the value of the data is a List of rows. The caller will have to call "instanceof" on the data value object if duplicates are expected, to determine if the data value of the Hashtable entry is a row or is a List of rows.

        Note, that for this routine to work efficiently the caller must ensure that the object in row[key_column_number] implements the hashCode and equals method as appropriate for it's datatype.

        It is expected that this call will be the first and only call made in an openscan. Qualifiers and stop position of the openscan are applied just as in a normal scan. This call is logically equivalent to the caller performing the following:

         import java.util.Hashtable;
        
         hash_table = new Hashtable();
        
         while (next())
         {
             row = create_new_row();
             fetch(row);
             if ((duplicate_value = 
                 hash_table.put(row[key_column_number], row)) != null)
             {
                 Vector row_vec;
        
                 // inserted a duplicate
                 if ((duplicate_value instanceof vector))
                 {
                     row_vec = (Vector) duplicate_value;
                 }
                 else
                 {
                     // allocate vector to hold duplicates
                     row_vec = new Vector(2);
        
                     // insert original row into vector
                     row_vec.addElement(duplicate_value);
        
                     // put the vector as the data rather than the row
                     hash_table.put(row[key_column_number], row_vec);
                 }
                 
                 // insert new row into vector
                 row_vec.addElement(row);
             }
         }
         

        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. RESOLVE - is this ok? or should I hard code somehow the row to be the first column and the row location?

        Currently it is only possible to hash on the first column in the conglomerate, in the future we may change the interface to allow hashing either on a different column or maybe on a combination of columns.

        No overflow to external storage is provided, so calling this routine on a 1 gigabyte conglomerate will incur at least 1 gigabyte of memory (probably failing with a java out of memory condition). If this routine gets an out of memory condition, or if "max_rowcnt" is exceeded then then the routine will give up, empty the Hashtable, and return "false."

        On exit from this routine, whether the fetchSet() succeeded or not the scan is complete, it is positioned just the same as if the scan had been drained by calling "next()" until it returns false (ie. fetchNext() and next() calls will return false). reopenScan() can be called to restart the scan.

        RESOLVE - until we get row counts what should we do for sizing the the size, capasity, and load factor of the hash table. For now it is up to the caller to create the Hashtable, Access does not reset any parameters.

        RESOLVE - I am not sure if access should be in charge of allocating the new row objects. I know that I can do this in the case of btree's, but I don't think I can do this in heaps. Maybe this is solved by work to be done on the sort interface.

        Specified by:
        fetchSet in interface ScanManager
        Parameters:
        max_rowcnt - The maximum number of rows to insert into the Hash table. Pass in -1 if there is no maximum.
        key_column_numbers - The column numbers of the columns in the scan result row to be the key to the Hashtable. "0" is the first column in the scan result row (which may be different than the first column in the row in the table of the scan).
        hash_table - The java HashTable to load into.
        Throws:
        StandardException - Standard exception policy.
      • reopenScan

        public void reopenScan​(DataValueDescriptor[] startKeyValue,
                               int startSearchOperator,
                               Qualifier[][] qualifier,
                               DataValueDescriptor[] stopKeyValue,
                               int stopSearchOperator)
                        throws StandardException
        Reposition the current scan. This call is semantically the same as if the current scan had been closed and a openScan() had been called instead. The scan is reopened with against the same conglomerate, and the scan is reopened with the same "hold" and "forUpdate" parameters passed in the original openScan. The previous template row continues to be used.
        Specified by:
        reopenScan in interface GenericScanController
        Parameters:
        startKeyValue - An indexable row which holds a (partial) key value which, in combination with the startSearchOperator, defines the starting position of the scan. If null, the starting position of the scan is the first row of the conglomerate.
        startSearchOperator - an operator which defines how the startKeyValue is to be searched for. If startSearchOperator is ScanController.GE, the scan starts on the first row which is greater than or equal to the startKeyValue. If startSearchOperation is ScanController.GT, the scan starts on the first row whose key is greater than startKeyValue. The startSearchOperation parameter is ignored if the startKeyValue parameter is null.
        qualifier - An array of qualifiers which, applied to each key, restrict the rows returned by the scan. Rows for which any one of the qualifiers returns false are not returned by the scan. If null, all rows are returned.
        stopKeyValue - An indexable row which holds a (partial) key value which, in combination with the stopSearchOperator, defines the ending position of the scan. If null, the ending position of the scan is the last row of the conglomerate.
        stopSearchOperator - an operator which defines how the stopKeyValue is used to determine the scan stopping position. If stopSearchOperation is ScanController.GE, the scan stops just before the first row which is greater than or equal to the stopKeyValue. If stopSearchOperation is ScanController.GT, the scan stops just before the first row whose key is greater than startKeyValue. The stopSearchOperation parameter is ignored if the stopKeyValue parameter is null.
        Throws:
        StandardException - Standard exception policy.
      • doesCurrentPositionQualify

        public boolean doesCurrentPositionQualify()
                                           throws StandardException
        Returns true if the current position of the scan still qualifies under the set of qualifiers passed to the openScan(). When called this routine will reapply all qualifiers against the row currently positioned and return true if the row still qualifies. If the row has been deleted or no longer passes the qualifiers then this routine will return false. This case can come about if the current scan or another scan on the same table in the same transaction deleted the row or changed columns referenced by the qualifier after the next() call which positioned the scan at this row. Note that for comglomerates which don't support update, like btree's, there is no need to recheck the qualifiers. The results of a fetch() performed on a scan positioned on a deleted row are undefined.
        Specified by:
        doesCurrentPositionQualify in interface ScanController
        Throws:
        StandardException - Standard exception policy.
      • fetch

        private void fetch​(DataValueDescriptor[] row,
                           boolean qualify)
                    throws StandardException
        Fetch the row at the current position of the Scan.
        Parameters:
        row - The row into which the value of the current position in the scan is to be stored.
        qualify - Indicates whether the qualifiers should be applied.
        Throws:
        StandardException - Standard exception policy.
      • getScanInfo

        public ScanInfo getScanInfo()
                             throws StandardException
        Return ScanInfo object which describes performance of scan.

        Return ScanInfo object which contains information about the current scan.

        Specified by:
        getScanInfo in interface GenericScanController
        Returns:
        The ScanInfo object which contains info about current scan.
        Throws:
        StandardException - Standard exception policy.
        See Also:
        ScanInfo
      • isCurrentPositionDeleted

        public boolean isCurrentPositionDeleted()
                                         throws StandardException
        Returns true if the current position of the scan is at a deleted row. This case can come about if the current scan or another scan on the same table in the same transaction deleted the row after the next() call which positioned the scan at this row. The results of a fetch() performed on a scan positioned on a deleted row are undefined.
        Specified by:
        isCurrentPositionDeleted in interface ScanController
        Throws:
        StandardException - Standard exception policy.