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/*! @page data_sources Data Sources
WiredTiger provides access to data from a variety of sources. At the
lowest level, data may be stored in a file using a tree structure. A
relational schema supporting tables, indices and column groups is
layered on top of file. Additional sources include LSM trees and
statistics, and applications can further extend the supported types by
implementing the ::WT_DATA_SOURCE interface.
Common operations on all data sources are performed using WT_CURSOR
handles. See @ref cursor_ops for a description of how to use
cursors.
@section data_builtin Builtin data sources
The following are the builtin cursor types:
<table>
@hrow{URI, Type, Key/Value types}
@row{<tt>backup:</tt>,
hot backup cursor,
key=<code>string</code>\, see @ref hot_backup for details}
@row{<tt>colgroup:\<table name\>:\<column group name\></tt>,
column group cursor,
table key\, column group value(s)}
@row{<tt>index:\<table name\>:\<index name\>[\<projection\>]</tt>,
index cursor,
key=index key\, value=table value(s) with optional projection
of columns}
@row{<tt>statistics:[\<data source URI\>]</tt>,
database or data source statistics cursor,
key=<code>int id</code>\, value=(<code>string description\,
string value\, uint64_t value</code>)\,
see @ref data_statistics for details}
@row{<tt>table:\<table name\>[\<projection\>]</tt>,
table cursor,
table key\, table value(s) with optional projection of columns}
</table>
Advanced applications may also open the following low-level cursor types:
<table>
@row{<tt>file:\<file name\></tt>,
file cursor,
file key\, file value(s)}
@row{<tt>lsm:\<name\></tt>,
LSM cursor (key=LSM key\, value=LSM value), See also: @ref lsm}
</table>
@subsection data_files Raw Files
WiredTiger's schema layer can be bypassed by opening cursors with a \c
"file:" URI, using the name of the underlying file. This can be useful for
seeing the contents of a column group or index without reading all of the
columns from the table.
For example, if an index becomes inconsistent with its primary, a file
cursor can read from the index without errors (even though some of the keys
that are returned may not exist in the primary).
@subsection data_indices Table Index data
When an index is created for a table, records are inserted into the index
whenever the table is updated. These records use a different key to the
primary table, as specified when the index is created with the
WT_SESSION::create method.
A cursor opened on an index has the specified index columns as its key,
accessed by WT_CURSOR::set_key and WT_CURSOR::get_key. The value columns
default to returning the value columns from the table, but this can be
overridden by configuring a projection cursor (see @ref cursor_projections),
which can access the table key columns or a subset of the value columns.
@subsection data_statistics Statistics Data
Statistics cursors can be used to retrieve run-time statistics about a
WiredTiger database as well as statistics for individual data sources.
The statistics are at two levels: per-database and per-individual data
source. Database-wide statistics are retrieved with the \c "statistics:"
URI; individual data source statistics are available by specifying
\c "statistics:<data source URI>".
The statistic key is an integer from the list of keys in @ref statistics_keys
"Statistics Keys". Statistics cursors return three values from the
WT_CURSOR::get_value call: a printable description of the statistic, a
printable version of the entry's value, and the entry's unsigned 64-bit
integral value, respectively.
The following is an example of printing run-time statistics about the
WiredTiger engine:
@snippet ex_stat.c statistics database function
The following is an example of printing statistics about a table:
@snippet ex_stat.c statistics table function
Both examples can use a common display routine that iterates through the
statistics until the cursor returns the end of the list.
@snippet ex_stat.c statistics display function
Individual statistics values can be retrieved by searching for the
corresponding key, as shown in the following example:
@snippet ex_stat.c statistics retrieve by key
See @ref tuning_statistics for more examples of how statistics can be used.
*/
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