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Release 5.2.28 on 6/10/2011
2011-09-13 17:44:24 +00:00
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<h2 class="title" style="clear: both"><a id="dbc_get"></a><span class="emphasis"><em>dbc</em></span> get</h2>
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<pre class="programlisting">dbc get
[-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 </pre>
<p>
The <span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span>
command returns a list of {key value} pairs, except in the case of the
<span class="bold"><strong>-get_recno</strong></span> and <span class="bold"><strong>-join_item</strong></span> options. In the case of the
<span class="bold"><strong>-get_recno</strong></span> option,
<span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span> returns
a list of the record number. In the case of the <span class="bold"><strong>-join_item</strong></span> option, <span class="emphasis"><em>dbc</em></span>
<span class="bold"><strong>get</strong></span> returns a list containing the
joined key.
</p>
<p>
The options follow, and are grouped by the action they perform.
</p>
<p>
The first group affects the position of the cursor in the database
without regard for the key or data item.
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<span class="bold"><strong>-current</strong></span>
</p>
<p>
Return the key/data pair to which the cursor currently refers.
</p>
<p>
If the cursor key/data pair was deleted, <span class="emphasis"><em>dbc</em></span>
<span class="bold"><strong>get</strong></span> will return an empty list.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-first</strong></span>
</p>
<p>
The cursor is set to refer to 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.
</p>
<p>
If the database is a Queue or Recno database, <span class="emphasis"><em>dbc</em></span>
<span class="bold"><strong>get</strong></span> using the <span class="bold"><strong>-first</strong></span> option will skip any keys that exist but
were never explicitly created by the application, or were created and
later deleted.
</p>
<p>
If the database is empty, <span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span> will return an empty list.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-last</strong></span>
</p>
<p>
The cursor is set to refer to 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.
</p>
<p>
If the database is a Queue or Recno database, <span class="emphasis"><em>dbc</em></span>
<span class="bold"><strong>get</strong></span> using the <span class="bold"><strong>-last</strong></span> option will skip any keys that exist but
were never explicitly created by the application, or were created and
later deleted.
</p>
<p>
If the database is empty, <span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span> will return an empty list.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-next</strong></span>
</p>
<p>
If the cursor is not yet initialized, the <span class="bold"><strong>-next</strong></span> option is identical to <span class="bold"><strong>-first</strong></span>.
</p>
<p>
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.
</p>
<p>
If the database is a Queue or Recno database, <span class="emphasis"><em>dbc</em></span>
<span class="bold"><strong>get</strong></span> using the <span class="bold"><strong>-next</strong></span> option will skip any keys that exist but
were never explicitly created by the application, or were created and
later deleted.
</p>
<p>
If the cursor is already on the last record in the database,
<span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span> will
return an empty list.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-nextdup</strong></span>
</p>
<p>
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,
<span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span> will
return an empty list.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-nextnodup</strong></span>
</p>
<p>
If the cursor is not yet initialized, the <span class="bold"><strong>-nextnodup</strong></span> option is identical to <span class="bold"><strong>-first</strong></span>.
</p>
<p>
Otherwise, the cursor is moved to the next non-duplicate key/data pair
of the database, and that pair is returned.
</p>
<p>
If no non-duplicate key/data pairs occur after the cursor position in
the database, <span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span> will return an empty list.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-prev</strong></span>
</p>
<p>
If the cursor is not yet initialized, <span class="bold"><strong>-prev</strong></span> is identical to <span class="bold"><strong>-last</strong></span>.
</p>
<p>
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.
</p>
<p>
If the database is a Queue or Recno database, <span class="emphasis"><em>dbc</em></span>
<span class="bold"><strong>get</strong></span> using the <span class="bold"><strong>-prev</strong></span> flag will skip any keys that exist but
were never explicitly created by the application, or were created and
later deleted.
</p>
<p>
If the cursor is already on the first record in the database,
<span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span> will
return an empty list.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-prevnodup</strong></span>
</p>
<p>
If the cursor is not yet initialized, the <span class="bold"><strong>-prevnodup</strong></span> option is identical to <span class="bold"><strong>-last</strong></span>.
</p>
<p>
Otherwise, the cursor is moved to the previous non-duplicate key/data
pair of the database, and that pair is returned.
</p>
<p>
If no non-duplicate key/data pairs occur before the cursor position in
the database, <span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span> will return an empty list.
</p>
</li>
</ul>
</div>
<p>
The next group of options are used to move the cursor to a
location in the database based on characteristics of the
key and/or data items.
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<span class="bold"><strong>-set</strong></span>
</p>
<p>
Move the cursor to the specified key/data pair of the database, and
return the datum associated with the given key.
</p>
<p>
In the presence of duplicate key values, <span class="emphasis"><em>dbc</em></span>
<span class="bold"><strong>get</strong></span> will return the first data item
for the given key.
</p>
<p>
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, <span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span> will return an empty list.
</p>
<p>
If no matching keys are found, <span class="emphasis"><em>dbc</em></span> <span class="bold"><strong>get</strong></span> will return an empty list.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-set_range</strong></span>
</p>
<p>
The <span class="bold"><strong>-set_range</strong></span> option is identical to
the <span class="bold"><strong>-set</strong></span> 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.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-get_both</strong></span>
</p>
<p>
The <span class="bold"><strong>-get_both</strong></span> option is identical to
the <span class="bold"><strong>-set</strong></span> option, except that both the
key and the data arguments must be matched by the key and data item in
the database.
</p>
<p>
For <span class="bold"><strong>-get_both</strong></span> to be specified, the
underlying database must be of type Btree or Hash.
</p>
</li>
</ul>
</div>
<p>
The last group of options is a general collection of
operations. Some of these involve positioning the cursor to
a location in the database based in information other than
what can be found in the key and/or data items. Others of
these have to do with the cursor's behavior upon retrieving
information.
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<span class="bold"><strong>-set_recno</strong></span>
</p>
<p>
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.
</p>
<p>
For the <span class="bold"><strong>-set_recno</strong></span> option to be
specified, the underlying database must be of type Btree, and it must
have been created with the <span class="bold"><strong>-recnum</strong></span>
option.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-get_recno</strong></span>
</p>
<p>
Return a list of the record number associated with the current cursor
position. No key argument should be specified.
</p>
<p>
For <span class="bold"><strong>-get_recno</strong></span> to be specified, the
underlying database must be of type Btree, and it must have been
created with the <span class="bold"><strong>-recnum</strong></span> option.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-join_item</strong></span>
</p>
<p>
Do not use the data value found in all 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.
</p>
<p>
For <span class="bold"><strong>-join_item</strong></span> to be specified, the
cursor must have been created by the <span class="emphasis"><em>db</em></span> <span class="bold"><strong>join</strong></span> command.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-partial {offset length}</strong></span>
</p>
<p>
The <span class="bold"><strong>dlen</strong></span> bytes starting <span class="bold"><strong>doff</strong></span> 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 any existing bytes are returned.
</p>
</li>
<li>
<p>
<span class="bold"><strong>-rmw</strong></span>
</p>
<p>
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.
</p>
</li>
</ul>
</div>
<p>
If a key is specified, and if the underlying database is a Queue or
Recno database, the given key will be interpreted by Tcl as an
integer. For all other database types, the key is interpreted by Tcl
as a byte array, unless indicated by a given option.
</p>
<p>
In the normal error case of attempting to retrieve a key that does not
exist an empty list is returned.
</p>
<p>
In the case of error, a Tcl error is thrown.
</p>
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