/*! @m_page{{c,java},statistics,Statistics}

WiredTiger can be configured to maintain a variety of run-time and
data-source statistics.  As maintaining statistics may involve updating
shared-memory data structures and traversing disk-based data structures,
configuring statistics may decrease application performance.

The ::wiredtiger_open \c statistics configuration must be set in order
for statistics to be maintained.  When configured to \c none (the
default), no statistics are maintained and attempting to read the
statistics will result in an error.   Alternatively, the \c fast
configuration maintains a subset of the statistics which are relatively
inexpensive, and the \c all configuration maintains all statistics
regardless of cost.

The following example configures WiredTiger to maintain all statistics,
regardless of cost:

@snippet ex_all.c Statistics configuration

Statistics are gathered and returned to the application using a statistics
cursor, which returns key/value pairs to the application.  See
@ref data_statistics for information about accessing the gathered statistics.

When the WT_SESSION::open_cursor method is called to open a statistics
cursor, the statistics configuration specifies the statistics to be
gathered, similarly to the ::wiredtiger_open function.   The \c fast
configuration gathers the subset of the statistics that are relatively
inexpensive, and the \c all configuration gathers all statistics,
regardless of cost.  If no configuration is specified, the current
database statistics configuration is assumed.

The configuration of the WT_SESSION::open_cursor method must agree with the
database configuration: if the database is configured to maintain \c fast
statistics, attempts to open a statistics cursor in \c all mode will fail.
If the database has been configured to maintain \c all statistics, the
statistics cursor can be configured in either \c fast or \c all modes.
For example, an application might configure the database to maintain all
available statistics, but the application might gather expensive statistics
less frequently than inexpensive ones.

The following example opens a statistics cursor on the database:

@snippet ex_all.c Statistics cursor database

The following example opens a statistics cursor on a table:

@snippet ex_all.c Statistics cursor table

The following example opens a statistics cursor on a table, but gathering
only the relatively inexpensive statistics:

@snippet ex_all.c Statistics cursor table fast

The WT_SESSION::open_cursor method and the ::wiredtiger_open function
also support the statistics configuration value \c clear.

When \c clear is specified to the WT_SESSION::open_cursor method, gathered
statistics will be reset, where appropriate, after they are gathered.  For
example, a cache size statistic is not cleared because it's not expected to
change rapidly over time, while the count of cursor insert operations will
be cleared.  This allows applications to easily monitor changes in the
system over time.

The following example gathers all statistics for a table, regardless of
cost, and then clears them:

@snippet ex_all.c Statistics cursor clear configuration

When \c clear is specified to the ::wiredtiger_open function, gathered
statistics will be reset, where appropriate, after they are gathered.
This applies to all statistics cursors (as if \c clear was configured
when the cursor was opened), as well as statistics logging, when it is
configured.

The following example configures WiredTiger to maintain only relatively
inexpensive statistics, and to clears statistics after they are gathered
or logged:

@snippet ex_all.c Statistics clear configuration

The following example opens a statistics cursor on an open join cursor:

@snippet ex_schema.c Statistics cursor join cursor

The statistics gathered will be organized by reference cursors participating
in the join (see WT_SESSION::join); the uri of each reference cursor appears
as a prefix in the description field returned as a value by the statistics
cursor.

@section statistics_log Statistics logging

WiredTiger will optionally log database statistics into files when the
the ::wiredtiger_open \c statistics_log configuration is set.

The log files are named \c WiredTiger.%%d.%%H, where \c %%d is replaced
with the day of the month as a decimal number (01-31), and \c %%H
is replaced by the hour (24-hour clock) as a decimal number (00-23).
Each log file contains the statistics for the hour specified in its name.

The location of the log files may be changed with the \c statistics_log.path
configuration string.

The following example logs statistics every 30 seconds:

@snippet ex_all.c Statistics logging

Each record is formatted as a space-separated timestamp, unsigned 64-bit
value and a variable length string which describes the statistic.

The timestamp format may be changed with the \c statistics_log.timestamp
configuration string.  The \c timestamp value may contain ISO C90 standard
strftime conversion specifications.

The statistics the database is configured to maintain are logged, that
is, configuring the database to maintain \c fast or \c all statistics
will modify the statistics that will be logged.

Statistics for specific underlying data sources may be included by adding
a list of data source URIs to the \c statistics_log configuration string:

@snippet ex_all.c Statistics logging with a table

Statistics for all underlying data sources of a particular type may be
included by adding a partial data source URI to the \c statistics_log
configuration string:

@snippet ex_all.c Statistics logging with a source type

When database statistics are logged, the database home will be the first
space-separated entry for each record in the log file.  For example:

@code
Mar 08 11:38:23 463 /database/home pthread mutex condition wait calls
Mar 08 11:38:23 0 /database/home files currently open
Mar 08 11:38:23 1855437 /database/home total heap memory allocations
Mar 08 11:38:23 1856622 /database/home total heap memory frees
Mar 08 11:38:23 1 /database/home total heap memory re-allocations
Mar 08 11:38:23 472 /database/home total read I/Os
@endcode

When data source statistics are logged, the data source's URI will be
the first space-separated entry for each record in the log file.  For
example:

@code
Mar 20 10:42:36 21 table:mytable compressed pages written
Mar 20 10:42:36 0 table:mytable page written failed to compress
Mar 20 10:42:36 5 table:mytable page written was too small to compress
Mar 20 10:42:36 586 table:mytable cursor insert calls
Mar 20 10:42:36 0 table:mytable bulk-loaded cursor-insert calls
@endcode

No statistics are logged for any data source for which a handle is not
currently open in the database, nor will any statistics requiring the
traversal of a tree (as if the \c statistics_fast configuration string
were set).

*/
