Common operations in WiredTiger are performed using Cursor handles. A cursor includes:
See Cursor operations in Java for a description of how to use cursors.
The following are the builtin basic cursor types:
|table cursor||table key, table value(s) with optional projection of columns|
|column group cursor||table key, column group value(s)|
|index cursor||key=index key, value=table value(s) with optional projection of columns|
|join cursor||key=table key, value=table value(s) with optional projection of columns|
Some administrative tasks can be accomplished using the following special cursor types that give access to data managed by WiredTiger:
see Log cursors for details
|metadata cursor (optionally only returning configuration strings for WT_SESSION::create if ||key=|
see Reading WiredTiger Metadata for details
|database||data source or join statistics cursor||key=|
see Statistics Data for details
Advanced applications may also open the following low-level cursor types:
|file cursor||file key, file value(s)|
|LSM cursor (key=LSM key, value=LSM value)||LSM key, LSM value,|
see Log-Structured Merge Trees
See the following for more details:
Cursors on tables and indices can return a subset of columns. This is done by listing the column names in parenthesis in the
uri parameter to Session.open_cursor. Only the fields from the listed columns are returned by Cursor.get_value.
The ex_schema.java example creates a table where the value format is
"5sHq", where the initial string is the country, the short is a year, and the long is a population. The following example lists just the country and year columns from the table record values:
This is particularly useful with index cursors, because if all columns in the projection are available in the index (including primary key columns, which are the values of the index), the data can be read from the index without accessing any column groups. See Index cursor projections for more information.
If there is a transaction active in a session, cursors operate in the context of that transaction. Reads performed while a transaction is active inherit the isolation level of the transaction, and updates performed within a transaction are made durable by calling Session.commit_transaction, or discarded by calling Session.rollback_transaction.
If no transaction is active, cursor reads are performed at the isolation level of the session, set with the
isolation configuration key to Connection.open_session and successful updates are automatically committed before the update operation completes.
Any operation that consists of multiple related updates should be enclosed in an explicit transaction to ensure that the updates are applied atomically.
read-committed (the default) or
snapshot isolation levels, committed changes from concurrent transactions become visible when no cursor is positioned. In other words, at these isolation levels, all cursors in a session read from a stable snapshot while any cursor in the session remains positioned.
Cursor positions survive transaction boundaries, unless a transaction is rolled back. When a transaction is rolled-back either implicitly or explicitly, all cursors in the session are reset as if the Cursor.reset method was called, discarding any cursor position as well as any key and value.
See Transactions in Java for more information.
Cursor positions hold resources that can inhibit the eviction of memory pages. If a cursor is active on a page being considered for eviction, the eviction will defer until the cursor is moved or reset. To avoid this and to keep resources freed in general, an application should call Cursor.reset during times it does not need to keep the cursor positioned. A cursor that has been reset is not active and will not inhibit eviction.
Cursors can be configured for raw mode by specifying the
"raw" config keyword to Session.open_cursor. In this mode, the methods Cursor.get_key, Cursor.get_value, Cursor.set_key and Cursor.set_value all take a single byte array in the variable-length argument list instead of a separate argument for each column.
The following example lists the table record values, using raw mode:
Raw mode can be used in combination with projections. The following example lists just the country and year columns from the table record values, using raw mode:
WiredTiger cursors provide access to data from a variety of sources. One of these sources is the list of objects in the database.
To retrieve the list of database objects, open a cursor on the uri
metadata:. Each returned key will be a database object and each returned value will be the information stored in the metadata for object named by the key.
To retrieve value strings that are valid arguments for calls to Session.create, open a cursor on
The metadata cursor is read-only, and the metadata cannot be modified.