org.apache.derby.iapi.store.access

Class BackingStoreHashtable

java.lang.Object

org.apache.derby.iapi.store.access.BackingStoreHashtable

public class BackingStoreHashtable
extends java.lang.Object

A BackingStoreHashtable is a utility class which will store a set of rows into an in memory hash table, or overflow the hash table to a tempory on disk structure.

All rows must contain the same number of columns, and the column at position N of all the rows must have the same format id. If the BackingStoreHashtable needs to be overflowed to disk, then an arbitrary row will be chosen and used as a template for creating the underlying overflow container.

The hash table will be built logically as follows (actual implementation may differ). The important points are that the hash value is the standard java hash value on the row[key_column_numbers[0], if key_column_numbers.length is 1, or row[key_column_numbers[0, 1, ...]] if key_column_numbers.length > 1, and that duplicate detection is done by the standard java duplicate detection provided by java.util.Hashtable.

import java.util.Hashtable;

hash_table = new Hashtable();

Object row; // is a DataValueDescriptor[] or a LocatedRow

boolean  needsToClone = rowSource.needsToClone();

while((row = rowSource.getNextRowFromRowSource()) != null)
{
    if (needsToClone)
        row = clone_row_from_row(row);

    Object key = KeyHasher.buildHashKey(row, key_column_numbers);

    if ((duplicate_value = 
        hash_table.put(key, 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(key, row_vec);
        }
        
        // insert new row into vector
        row_vec.addElement(row);
    }
}

What actually goes into the hash table is a little complicated. That is because the row may either be an array of column values (i.e. DataValueDescriptor[]) or a LocatedRow (i.e., a structure holding the columns plus a RowLocation). In addition, the hash value itself may either be one of these rows or (in the case of multiple rows which hash to the same value) a bucket (List) of rows. To sum this up, the values in a hash table which does not spill to disk may be the following:

• DataValueDescriptor[] and ArrayList
• or LocatedRow and ArrayList

If rows spill to disk, then they just become arrays of columns. In this case, a LocatedRow becomes a DataValueDescriptor[], where the last cell contains the RowLocation.

Constructor Summary

Constructors

Constructor and Description
BackingStoreHashtable(TransactionController tc, RowSource row_source, int[] key_column_numbers, boolean remove_duplicates, long estimated_rowcnt, long max_inmemory_rowcnt, int initialCapacity, float loadFactor, boolean skipNullKeyColumns, boolean keepAfterCommit) Create the BackingStoreHashtable from a row source.

Method Summary

Modifier and Type	Method and Description
void	close() Close the BackingStoreHashtable.
java.util.Enumeration<java.lang.Object>	elements() Return an Enumeration that can be used to scan the entire table.
java.lang.Object	get(java.lang.Object key) Get data associated with given key.
void	getAllRuntimeStats(java.util.Properties prop) Return runtime stats to caller by adding them to prop.
boolean	includeRowLocations() Return true if we should include RowLocations with the rows stored in this hash table.
boolean	putRow(boolean needsToClone, DataValueDescriptor[] row, RowLocation rowLocation) Put a row into the hash table.
java.lang.Object	remove(java.lang.Object key) remove a row from the hash table.
void	setAuxillaryRuntimeStats(java.util.Properties prop) Set the auxillary runtime stats.
int	size() Return number of unique rows in the hash table.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

BackingStoreHashtable

public BackingStoreHashtable(TransactionController tc,
                             RowSource row_source,
                             int[] key_column_numbers,
                             boolean remove_duplicates,
                             long estimated_rowcnt,
                             long max_inmemory_rowcnt,
                             int initialCapacity,
                             float loadFactor,
                             boolean skipNullKeyColumns,
                             boolean keepAfterCommit)
                      throws StandardException

Create the BackingStoreHashtable from a row source.

This routine drains the RowSource. The performance characteristics depends on the number of rows inserted and the parameters to the constructor. RowLocations are supported iff row_source is null. RowLocations in a non-null row_source can be added later if there is a use-case that stresses this behavior.

If the number of rows is <= "max_inmemory_rowcnt", then the rows are inserted into a java.util.HashMap. In this case no TransactionController is necessary, a "null" tc is valid.

If the number of rows is > "max_inmemory_rowcnt", then the rows will be all placed in some sort of Access temporary file on disk. This case requires a valid TransactionController.

Parameters:

tc - An open TransactionController to be used if the hash table needs to overflow to disk.

row_source - RowSource to read rows from.

key_column_numbers - The column numbers of the columns in the scan result row to be the key to the HashMap. "0" is the first column in the scan result row (which may be different than the first row in the table of the scan).

remove_duplicates - Should the HashMap automatically remove duplicates, or should it create the list of duplicates?

estimated_rowcnt - The estimated number of rows in the hash table. Pass in -1 if there is no estimate.

max_inmemory_rowcnt - The maximum number of rows to insert into the inmemory Hash table before overflowing to disk. Pass in -1 if there is no maximum.

initialCapacity - If not "-1" used to initialize the java HashMap

loadFactor - If not "-1" used to initialize the java HashMap

skipNullKeyColumns - Skip rows with a null key column, if true.

keepAfterCommit - If true the hash table is kept after a commit, if false the hash table is dropped on the next commit.

Throws:

StandardException - Standard exception policy.

Method Detail

includeRowLocations

public boolean includeRowLocations()

Return true if we should include RowLocations with the rows stored in this hash table.

close

public void close()
           throws StandardException

Close the BackingStoreHashtable.

Perform any necessary cleanup after finishing with the hashtable. Will deallocate/dereference objects as necessary. If the table has gone to disk this will drop any on disk files used to support the hash table.

Throws:

StandardException - Standard exception policy.

elements

public java.util.Enumeration<java.lang.Object> elements()
                                                 throws StandardException

Return an Enumeration that can be used to scan the entire table. The objects in the Enumeration can be either of the following:

• a row - This is a single row with a unique hash key.
• or a bucket of rows - This is a list of rows which all have the same hash key.

The situation is a little more complicated because the row representation is different depending on whether the row includes a RowLocation. If includeRowLocations()== true, then the row is a LocatedRow. Otherwise, the row is an array of DataValueDescriptor. Putting all of this together, if the row contains a RowLocation, then the objects in the Enumeration returned by this method can be either of the following:

• a LocatedRow
• or a List<LocatedRow>

But if the row does not contain a RowLocation, then the objects in the Enumeration returned by this method can be either of the following:

• a DataValueDescriptor[]
• or a List<DataValueDescriptor[]>

RESOLVE - is it worth it to support this routine when we have a disk overflow hash table?

Returns:

The Enumeration.

Throws:

StandardException - Standard exception policy.

get

public java.lang.Object get(java.lang.Object key)
                     throws StandardException

Get data associated with given key.

There are 2 different types of objects returned from this routine.

In both cases, the key value is either the object stored in row[key_column_numbers[0]], if key_column_numbers.length is 1, otherwise it is a KeyHasher containing the objects stored in row[key_column_numbers[0, 1, ...]]. For every qualifying unique row value an entry is placed into the hash table.

For row values with duplicates, the value of the data is a list of rows.

The situation is a little more complicated because the row representation is different depending on whether the row includes a RowLocation. If includeRowLocations() == true, then the row is a LocatedRow. Otherwise, the row is an array of DataValueDescriptor. Putting all of this together, if the row contains a RowLocation, then the objects returned by this method can be either of the following:

• a LocatedRow
• or a List<LocatedRow>

But if the row does not contain a RowLocation, then the objects returned by this method can be either of the following:

• a DataValueDescriptor[]
• or a List<DataValueDescriptor[]>

The caller will have to call "instanceof" on the data value object if duplicates are expected, to determine if the data value of the hash table entry is a row or is a list of rows.

See the javadoc for elements() for more information on the objects returned by this method.

The BackingStoreHashtable "owns" the objects returned from the get() routine. They remain valid until the next access to the BackingStoreHashtable. If the client needs to keep references to these objects, it should clone copies of the objects. A valid BackingStoreHashtable can place all rows into a disk based conglomerate, declare a row buffer and then reuse that row buffer for every get() call.

Parameters:

key - The key to hash on.

Returns:

The value to which the key is mapped in this hashtable; null if the key is not mapped to any value in this hashtable.

Throws:

StandardException - Standard exception policy.

getAllRuntimeStats

public void getAllRuntimeStats(java.util.Properties prop)
                        throws StandardException

Return runtime stats to caller by adding them to prop.

Parameters:

prop - The set of properties to append to.

Throws:

StandardException - Standard exception policy.

remove

public java.lang.Object remove(java.lang.Object key)
                        throws StandardException

remove a row from the hash table.

a remove of a duplicate removes the entire duplicate list.

Parameters:

key - The key of the row to remove.

Throws:

StandardException - Standard exception policy.

setAuxillaryRuntimeStats

public void setAuxillaryRuntimeStats(java.util.Properties prop)
                              throws StandardException

Set the auxillary runtime stats.

getRuntimeStats() will return both the auxillary stats and any BackingStoreHashtable() specific stats. Note that each call to setAuxillaryRuntimeStats() overwrites the Property set that was set previously.

Parameters:

prop - The set of properties to append from.

Throws:

StandardException - Standard exception policy.

putRow

public boolean putRow(boolean needsToClone,
                      DataValueDescriptor[] row,
                      RowLocation rowLocation)
               throws StandardException

Put a row into the hash table.

The in memory hash table will need to keep a reference to the row after the put call has returned. If "needsToClone" is true then the hash table will make a copy of the row and put that, else if "needsToClone" is false then the hash table will keep a reference to the row passed in and no copy will be made.

If routine returns false, then no reference is kept to the duplicate row which was rejected (thus allowing caller to reuse the object).

Parameters:

needsToClone - does this routine have to make a copy of the row, in order to keep a reference to it after return?

row - The row to insert into the table.

rowLocation - Location of row in conglomerate; could be null.

Returns:

true if row was inserted into the hash table. Returns false if the BackingStoreHashtable is eliminating duplicates, and the row being inserted is a duplicate, or if we are skipping rows with 1 or more null key columns and we find a null key column.

Throws:

StandardException - Standard exception policy.

size

public int size()
         throws StandardException

Return number of unique rows in the hash table.

Returns:

The number of unique rows in the hash table.

Throws:

StandardException - Standard exception policy.