WT_CURSOR Struct Reference

A WT_CURSOR handle is the interface to a cursor. More...

Public Member Functions
int	close (WT_CURSOR *cursor)
	Close the cursor. More...
Data access
int	get_key (WT_CURSOR *cursor,...)
	Get the key for the current record. More...
int	get_value (WT_CURSOR *cursor,...)
	Get the value for the current record. More...
void	set_key (WT_CURSOR *cursor,...)
	Set the key for the next operation. More...
void	set_value (WT_CURSOR *cursor,...)
	Set the value for the next operation. More...
Cursor positioning
int	compare (WT_CURSOR *cursor, WT_CURSOR *other, int *comparep)
	Return the ordering relationship between two cursors: both cursors must have the same data source and have valid keys. More...
int	next (WT_CURSOR *cursor)
	Return the next record. More...
int	prev (WT_CURSOR *cursor)
	Return the previous record. More...
int	reset (WT_CURSOR *cursor)
	Reset the position of the cursor. More...
int	search (WT_CURSOR *cursor)
	Return the record matching the key. More...
int	search_near (WT_CURSOR *cursor, int *exactp)
	Return the record matching the key if it exists, or an adjacent record. More...
Data modification
int	insert (WT_CURSOR *cursor)
	Insert a record and optionally update an existing record. More...
int	update (WT_CURSOR *cursor)
	Update a record and optionally insert an existing record. More...
int	remove (WT_CURSOR *cursor)
	Remove a record. More...

Public Attributes
WT_SESSION *	session
	The session handle for this cursor. More...
const char *	uri
	The name of the data source for the cursor, matches the uri parameter to WT_SESSION::open_cursor used to open the cursor.
const char *	key_format
	The format of the data packed into key items. More...
const char *	value_format
	The format of the data packed into value items. More...

Detailed Description

A WT_CURSOR handle is the interface to a cursor.

Cursors allow data to be searched, iterated and modified, implementing the CRUD (create, read, update and delete) operations. Cursors are opened in the context of a session. If a transaction is started, cursors operate in the context of the transaction until the transaction is resolved.

Raw data is represented by key/value pairs of WT_ITEM structures, but cursors can also provide access to fields within the key and value if the formats are described in the WT_SESSION::create method.

In the common case, a cursor is used to access records in a table. However, cursors can be used on subsets of tables (such as a single column or a projection of multiple columns), as an interface to statistics, configuration data or application-specific data sources. See WT_SESSION::open_cursor for more information.

Thread safety: A WT_CURSOR handle is not usually shared between threads, see Multithreading for more information.

Examples:

ex_access.c, ex_call_center.c, ex_config.c, ex_cursor.c, ex_schema.c, ex_stat.c, and ex_thread.c.

Member Function Documentation

int WT_CURSOR::close

(

WT_CURSOR *

cursor

)

Close the cursor.

This releases the resources associated with the cursor handle. Cursors are closed implicitly by ending the enclosing connection or closing the session in which they were opened.

        ret = cursor->close(cursor);

Parameters

cursor

the cursor handle

Returns

zero on success and a non-zero error code on failure. See Error Returns for details.

Examples:

ex_call_center.c, ex_cursor.c, ex_schema.c, and ex_stat.c.

int WT_CURSOR::compare	(	WT_CURSOR *	cursor,
		WT_CURSOR *	other,
		int *	comparep
	)

Return the ordering relationship between two cursors: both cursors must have the same data source and have valid keys.

        int compare;
        ret = cursor->compare(cursor, other, &compare);
        if (compare == 0) {
                /* Cursors reference the same key */
        } else if (compare < 0) {
                /* Cursor key less than other key */
        } else if (compare > 0) {
                /* Cursor key greater than other key */
        }

Parameters

cursor	the cursor handle
other	another cursor handle
comparep	the status of the comparison: < 0 if cursor refers to a key that appears before other, 0 if the cursors refer to the same key, and > 0 if cursor refers to a key that appears after other.

Returns

zero on success and a non-zero error code on failure. See Error Returns for details.

int WT_CURSOR::get_key	(	WT_CURSOR *	cursor,
			...
	)

Get the key for the current record.

        const char *key;        /* Get the cursor's string key. */
        ret = cursor->get_key(cursor, &key);

        uint64_t recno;         /* Get the cursor's record number key. */
        ret = cursor->get_key(cursor, &recno);

Parameters

cursor	the cursor handle
...	pointers to hold key fields corresponding to WT_CURSOR::key_format.

Returns

zero on success and a non-zero error code on failure. See Error Returns for details.

Examples:

ex_access.c, ex_config.c, ex_cursor.c, ex_schema.c, and ex_thread.c.

int WT_CURSOR::get_value	(	WT_CURSOR *	cursor,
			...
	)

Get the value for the current record.

        const char *value;      /* Get the cursor's string value. */
        ret = cursor->get_value(cursor, &value);

        WT_ITEM value;          /* Get the cursor's raw value. */
        ret = cursor->get_value(cursor, &value);

Parameters

cursor	the cursor handle
...	pointers to hold value fields corresponding to WT_CURSOR::value_format.

Returns

zero on success and a non-zero error code on failure. See Error Returns for details.

Examples:

ex_access.c, ex_call_center.c, ex_config.c, ex_cursor.c, ex_schema.c, ex_stat.c, and ex_thread.c.

int WT_CURSOR::insert

(

WT_CURSOR *

cursor

)

Insert a record and optionally update an existing record.

If the cursor was configured with "overwrite=true" (the default), both the key and value must be set; if the record already exists, the key's value will be updated, otherwise, the record will be inserted.

        /* Insert a new record or overwrite an existing record. */
        const char *key = "some key", *value = "some value";
        ret = session->open_cursor(
            session, "table:mytable", NULL, NULL, &cursor);
        cursor->set_key(cursor, key);
        cursor->set_value(cursor, value);
        ret = cursor->insert(cursor);

If the cursor was not configured with "overwrite=true", both the key and value must be set and the record must not already exist; the record will be inserted.

        /* Insert a new record and fail if the record exists. */
        const char *key = "some key", *value = "some value";
        ret = session->open_cursor(
            session, "table:mytable", NULL, "overwrite=false", &cursor);
        cursor->set_key(cursor, key);
        cursor->set_value(cursor, value);
        ret = cursor->insert(cursor);

If a cursor with record number keys was configured with "append=true" (not the default), the value must be set; a new record will be appended and the record number set as the cursor key value.

        /* Insert a new record and assign a record number. */
        uint64_t recno;
        const char *value = "some value";
        ret = session->open_cursor(
            session, "table:mytable", NULL, "append", &cursor);
        cursor->set_value(cursor, value);
        ret = cursor->insert(cursor);
        if (ret == 0)
                ret = cursor->get_key(cursor, &recno);

The cursor ends with no position, and a subsequent call to the WT_CURSOR::next (WT_CURSOR::prev) method will iterate from the beginning (end) of the table.

Inserting a new record after the current maximum record in a fixed-length bit field column-store (that is, a store with an 'r' type key and 't' type value) may implicitly create the missing records as records with a value of 0.

When loading a large amount of data into a new object, using a cursor with the bulk configuration string enabled and loading the data in sorted order will be much faster than doing out-of-order inserts. See Bulk-load for more information.

Parameters

cursor

the cursor handle

Returns

zero on success and a non-zero error code on failure. See Error Returns for details. In particular, if overwrite is not configured and a record with the specified key already exists, WT_DUPLICATE_KEY is returned.

Examples:

ex_access.c, ex_call_center.c, ex_cursor.c, ex_schema.c, ex_stat.c, and ex_thread.c.

int WT_CURSOR::next

(

WT_CURSOR *

cursor

)

Return the next record.

        ret = cursor->next(cursor);

Parameters

cursor

the cursor handle

Returns

zero on success and a non-zero error code on failure. See Error Returns for details.

Examples:

ex_access.c, ex_config.c, ex_cursor.c, ex_schema.c, ex_stat.c, and ex_thread.c.

int WT_CURSOR::prev

(

WT_CURSOR *

cursor

)

Return the previous record.

        ret = cursor->prev(cursor);

Parameters

cursor

the cursor handle

Returns

zero on success and a non-zero error code on failure. See Error Returns for details.

Examples:

ex_call_center.c, and ex_cursor.c.

int WT_CURSOR::remove

(

WT_CURSOR *

cursor

)

Remove a record.

If the cursor was configured with "overwrite=true" (the default), the key must be set; the key's record will be removed if it exists, no error will be returned if the record does not exist.

        const char *key = "some key";
        ret = session->open_cursor(
            session, "table:mytable", NULL, NULL, &cursor);
        cursor->set_key(cursor, key);
        ret = cursor->remove(cursor);

If the cursor was not configured with "overwrite=true", the key must be set and the key's record must exist; the record will be removed.

        const char *key = "some key";
        ret = session->open_cursor(
            session, "table:mytable", NULL, "overwrite=false", &cursor);
        cursor->set_key(cursor, key);
        ret = cursor->remove(cursor);

Removing a record in a fixed-length bit field column-store (that is, a store with an 'r' type key and 't' type value) is identical to setting the record's value to 0.

On success, the cursor ends positioned at the removed record; to minimize cursor resources, the WT_CURSOR::reset method should be called as soon as the cursor no longer needs that position.

Parameters

cursor

the cursor handle

Returns

zero on success and a non-zero error code on failure. See Error Returns for details. In particular, if overwrite is not configured and no record with the specified key exists, WT_NOTFOUND is returned.

Examples:

ex_cursor.c.

int WT_CURSOR::reset

(

WT_CURSOR *

cursor

)

Reset the position of the cursor.

Any resources held by the cursor are released, and the cursor's key and position are no longer valid. A subsequent iteration with WT_CURSOR::next will move to the first record, or with WT_CURSOR::prev will move to the last record.

        ret = cursor->reset(cursor);

Parameters

cursor

the cursor handle

Returns

zero on success and a non-zero error code on failure. See Error Returns for details.

Examples:

ex_access.c, and ex_cursor.c.

int WT_CURSOR::search

(

WT_CURSOR *

cursor

)

Return the record matching the key.

The key must first be set.

        const char *key = "some key";
        cursor->set_key(cursor, key);
        ret = cursor->search(cursor);

On success, the cursor ends positioned at the returned record; to minimize cursor resources, the WT_CURSOR::reset method should be called as soon as the record has been retrieved and the cursor no longer needs that position.

Parameters

cursor

the cursor handle

Returns

zero on success and a non-zero error code on failure. See Error Returns for details.

Examples:

ex_call_center.c, ex_cursor.c, ex_schema.c, and ex_stat.c.

int WT_CURSOR::search_near	(	WT_CURSOR *	cursor,
		int *	exactp
	)

Return the record matching the key if it exists, or an adjacent record.

An adjacent record is either the smallest record larger than the key or the largest record smaller than the key (in other words, a logically adjacent key).

The key must first be set.

An example of a search for an exact or adjacent match:

        cursor->set_key(cursor, key);
        ret = cursor->search_near(cursor, &exact);
        if (ret == 0) {
                if (exact == 0) {
                        /* an exact match */
                } else if (exact < 0) {
                        /* returned smaller key */
                } else if (exact > 0) {
                        /* returned larger key */
                }
        }

An example of a forward scan through the table, where all keys greater than or equal to a specified prefix are included in the scan:

        cursor->set_key(cursor, key);
        ret = cursor->search_near(cursor, &exact);
        if (ret == 0 && exact >= 0) {
                /* include first key returned in the scan */
        }

        while ((ret = cursor->next(cursor)) == 0) {
                /* the rest of the scan */
        }

An example of a backward scan through the table, where all keys less than a specified prefix are included in the scan:

        cursor->set_key(cursor, key);
        ret = cursor->search_near(cursor, &exact);
        if (ret == 0 && exact < 0) {
                /* include first key returned in the scan */
        }

        while ((ret = cursor->prev(cursor)) == 0) {
                /* the rest of the scan */
        }

On success, the cursor ends positioned at the returned record; to minimize cursor resources, the WT_CURSOR::reset method should be called as soon as the record has been retrieved and the cursor no longer needs that position.

Parameters

cursor	the cursor handle
exactp	the status of the search: 0 if an exact match is found, < 0 if a smaller key is returned, > 0 if a larger key is returned

Returns

zero on success and a non-zero error code on failure. See Error Returns for details.

Examples:

ex_call_center.c, and ex_cursor.c.

void WT_CURSOR::set_key	(	WT_CURSOR *	cursor,
			...
	)

Set the key for the next operation.

                                /* Set the cursor's string key. */
        const char *key = "another key";
        cursor->set_key(cursor, key);

        uint64_t recno = 37;    /* Set the cursor's record number key. */
        cursor->set_key(cursor, recno);

Parameters

cursor	the cursor handle
...	key fields corresponding to WT_CURSOR::key_format.

If an error occurs during this operation, a flag will be set in the cursor, and the next operation to access the key will fail. This simplifies error handling in applications.

Examples:

ex_access.c, ex_call_center.c, ex_cursor.c, ex_schema.c, ex_stat.c, and ex_thread.c.

void WT_CURSOR::set_value	(	WT_CURSOR *	cursor,
			...
	)

Set the value for the next operation.

                                /* Set the cursor's string value. */
        const char *value = "another value";
        cursor->set_value(cursor, value);

        WT_ITEM value;          /* Set the cursor's raw value. */
        value.data = "another value";
        value.size = strlen("another value");
        cursor->set_value(cursor, &value);

Parameters

cursor	the cursor handle
...	value fields corresponding to WT_CURSOR::value_format.

If an error occurs during this operation, a flag will be set in the cursor, and the next operation to access the value will fail. This simplifies error handling in applications.

Examples:

ex_access.c, ex_call_center.c, ex_cursor.c, ex_schema.c, ex_stat.c, and ex_thread.c.

int WT_CURSOR::update

(

WT_CURSOR *

cursor

)

Update a record and optionally insert an existing record.

If the cursor was configured with "overwrite=true" (the default), both the key and value must be set; if the record already exists, the key's value will be updated, otherwise, the record will be inserted.

        const char *key = "some key", *value = "some value";
        ret = session->open_cursor(
            session, "table:mytable", NULL, NULL, &cursor);
        cursor->set_key(cursor, key);
        cursor->set_value(cursor, value);
        ret = cursor->update(cursor);

If the cursor was not configured with "overwrite=true", both the key and value must be set and the record must already exist; the record will be updated.

        const char *key = "some key", *value = "some value";
        ret = session->open_cursor(
            session, "table:mytable", NULL, "overwrite=false", &cursor);
        cursor->set_key(cursor, key);
        cursor->set_value(cursor, value);
        ret = cursor->update(cursor);

On success, the cursor ends positioned at the modified record; to minimize cursor resources, the WT_CURSOR::reset method should be called as soon as the cursor no longer needs that position.

Parameters

cursor

the cursor handle

Returns

zero on success and a non-zero error code on failure. See Error Returns for details. In particular, if overwrite is not configured and no record with the specified key exists, WT_NOTFOUND is returned.

Examples:

ex_cursor.c.

Member Data Documentation

const char* WT_CURSOR::key_format

The format of the data packed into key items.

See Packing and Unpacking Data for details. If not set, a default value of "u" is assumed, and applications must use WT_ITEM structures to manipulate untyped byte arrays.

WT_SESSION* WT_CURSOR::session

The session handle for this cursor.

const char* WT_CURSOR::value_format

The format of the data packed into value items.

See Packing and Unpacking Data for details. If not set, a default value of "u" is assumed, and applications must use WT_ITEM structures to manipulate untyped byte arrays.