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 Cursor operations for a description of how to use cursors.

Builtin data sources

The following are the builtin cursor types:

URI	Type
backup:	hot backup cursor
colgroup:<tablename>.<columnset>	column group cursor
config:[<uri>]	object configuration cursor (key=config string, value=config value)
table:<tablename>	table cursor (key=table key, value=table value)
file:<filename>	file cursor (key=file key, value=file value)
index:<tablename>.<indexname>	index cursor (key=index key, value=table value)
join:<cursor1>&<cursor2>[&<cursor3>...]	join cursor Not yet supported in WiredTiger.
lsm:<name>	LSM cursor (key=LSM key, value=LSM value)	see Log-Structured Merge Trees
statistics:[file:<filename>]	database or file statistics (key=(int), value=(string)description, (string)value, (uint64_t)value)

Hot backup cursors

See Hot backup for more information.

Raw Files

WiredTiger's schema layer can be bypassed by opening cursors with a "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).

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 Projections), which can access the table key columns or a subset of the value columns.

Statistics Data

Cursors can return run-time statistics about the WiredTiger engine as well as statistics for the underlying row- and column-store files. Each cursor iteration sets three separate values: a printable description of the entry, a printable version of the entry's value, and the entry's unsigned 64-bit integral value.

The statistic key is an integer from the list of keys in Statistics Keys.

The following is an example of printing run-time statistics about the WiredTiger engine:

        if ((ret = session->open_cursor(session,
            "statistics:", NULL, NULL, &cursor)) != 0)
                return (ret);

        return (print_cursor(cursor));

The following is an example of printing statistics about an underlying file:

        if ((ret = session->open_cursor(session,
            "statistics:file:access.wt", NULL, NULL, &cursor)) != 0)
                return (ret);

        return (print_cursor(cursor));

Both examples can use a common display routine that iterates through the statistics until the cursor returns the end of the list.

int
print_cursor(WT_CURSOR *cursor)
{
        const char *desc, *pvalue;
        uint64_t value;
        int ret;

        while (
            (ret = cursor->next(cursor)) == 0 &&
            (ret = cursor->get_value(cursor, &desc, &pvalue, &value)) == 0)
                printf("%s=%s\n", desc, pvalue);

        return (ret == WT_NOTFOUND ? 0 : ret);
}