Berkeley DB: db get
Description: Berkeley DB: An embedded database programmatic toolkit.
Keywords: embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++

# dbc get |

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    dbc get
    	[-consume]
    	[-current]
    	[-first]
    	[-get_recno]
    	[-join_item]
    	[-last]
    	[-next]
    	[-nextdup]
    	[-nextnodup]
    	[-partial {offset length}]
    	[-prev]
    	[-prevnodup]
    	[-rmw]
    dbc get
    	[-partial {offset length}]
    	[-rmw]
    	[-set]
    	[-set_range]
    	[-set_recno]
    	key
    dbc get
    	-get_both
    	[-partial {offset length}]
    	[-rmw]
    	key data

# Description

The dbc get command returns a list of {key value} pairs, except in the case of the -get_recno and -join_item options. In the case of the -get_recno option, dbc get returns a list of the record number. In the case of the -join_item option, dbc get returns a list containing the joined key.

The options are as follows:

-consume
  Return the record number and data from the available record closest to the head of the queue and delete the record. The cursor will be positioned on the deleted record.

  A record is available if it is not deleted and is not currently locked. For the -consume option to be specified, the underlying database must be of type Queue.

-current
  Return the key/data pair currently referenced by the cursor.

  If the cursor key/data pair was deleted, dbc get will return an empty list.

-first
  The cursor is set to reference the first key/data pair of the database, and that pair is returned. In the presence of duplicate key values, the first data item in the set of duplicates is returned.

  If the database is a Queue or Recno database, dbc get using the -first option will skip any keys that exist but were never explicitly created by the application or were created and later deleted.

  If the database is empty, dbc get will return an empty list.

-last
  The cursor is set to reference the last key/data pair of the database, and that pair is returned. In the presence of duplicate key values, the last data item in the set of duplicates is returned.

  If the database is a Queue or Recno database, dbc get using the -last option will skip any keys that exist but were never explicitly created by the application or were created and later deleted.

  If the database is empty, dbc get will return an empty list.

-next
  If the cursor is not yet initialized, the -next option is identical to -first.

  Otherwise, the cursor is moved to the next key/data pair of the database, and that pair is returned. In the presence of duplicate key values, the value of the key may not change.

  If the database is a Queue or Recno database, dbc get using the -next option will skip any keys that exist but were never explicitly created by the application or were created and later deleted.

  If the cursor is already on the last record in the database, dbc get will return an empty list.

-nextdup
  If the next key/data pair of the database is a duplicate record for the current key/data pair, the cursor is moved to the next key/data pair of the database, and that pair is returned. Otherwise, dbc get will return an empty list.

-nextnodup
  If the cursor is not yet initialized, the -nextnodup option is identical to -first.

  Otherwise, the cursor is moved to the next non-duplicate key/data pair of the database, and that pair is returned.

  If no non-duplicate key/data pairs occur after the cursor position in the database, dbc get will return an empty list.

-prev
  If the cursor is not yet initialized, -prev is identical to -last.

  Otherwise, the cursor is moved to the previous key/data pair of the database, and that pair is returned. In the presence of duplicate key values, the value of the key may not change.

  If the database is a Queue or Recno database, dbc get using the -prev flag will skip any keys that exist but were never explicitly created by the application or were created and later deleted.

  If the cursor is already on the first record in the database, dbc get will return an empty list.

-prevnodup
  If the cursor is not yet initialized, the -prevnodup option is identical to -last.

  Otherwise, the cursor is moved to the previous non-duplicate key/data pair of the database, and that pair is returned.

  If no non-duplicate key/data pairs occur before the cursor position in the database, dbc get will return an empty list.

-set
  Move the cursor to the specified key/data pair of the database, and return the datum associated with the given key.

  In the presence of duplicate key values, dbc get will return the first data item for the given key.

  If the database is a Queue or Recno database and the requested key exists, but was never explicitly created by the application or was later deleted, dbc get will return an empty list.

  If no matching keys are found, dbc get will return an empty list.

-set_range
  The -set_range option is identical to the -set option, except that the key is returned as well as the data item, and, in the case of the Btree access method, the returned key/data pair is the smallest key greater than or equal to the specified key (as determined by the comparison function), permitting partial key matches and range searches.

-get_both
  The -get_both option is identical to the -set option, except that both the key and the data arguments must be matched by the key and data item in the database.

  For -get_both to be specified, the underlying database must be of type Btree or Hash.

-set_recno
  Move the cursor to the specific numbered record of the database, and return the associated key/data pair. The key must be a record number.

  For the -set_recno option to be specified, the underlying database must be of type Btree and it must have been created with the -recnum option.

-get_recno
  Return a list of the record number associated with the current cursor position. No key argument should be specified.

  For -get_recno to be specified, the underlying database must be of type Btree and it must have been created with the -recnum option.

-join_item
  Do not use the data value found in all of the cursors as a lookup key for the primary database, but simply return it in the key parameter instead. The data parameter is left unchanged.

  For -join_item to be specified, the cursor must have been created by the db join command.

-partial {offset length}
  The dlen bytes starting doff bytes from the beginning of the retrieved data record are returned as if they comprised the entire record. If any or all of the specified bytes do not exist in the record, the command is successful and the existing bytes or 0 bytes are returned.

-rmw
  Acquire write locks instead of read locks when doing the retrieval. Setting this flag may decrease the likelihood of deadlock during a read-modify-write cycle by immediately acquiring the write lock during the read part of the cycle so that another thread of control acquiring a read lock for the same item, in its own read-modify-write cycle, will not result in deadlock.

In the normal error case of attempting to retrieve a key that does not exist an empty list is returned.

In the case of error, a Tcl error is thrown.

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