/*! @m_page{{c,java},schema,Schema, Columns, Column Groups, Indices and Projections}
While many tables have simple key/value pairs for records, WiredTiger
also supports more complex data patterns.
@section schema_intro Tables, rows and columns
A table is a logical representation of data consisting of cells in rows
and columns. For example, a database might have a simple table
including an employee identifier, last name and first name, and a
salary:
@hrow{Employee ID, Last Name, First Name, Salary}
@row{1, Smith, Joe, 40000}
@row{2, Jones, Mary, 50000}
@row{3, Johnson, Cathy, 44000}
A row-oriented database would store all of the values for the first
employee in the first row, then the next employee's values in the next
row, and so on:
1,Smith,Joe,40000
2,Jones,Mary,50000
3,Johnson,Cathy,44000
A column-oriented database would store all of the values of a column
together, then the values of the next column, and so on:
1,2,3
Smith,Jones,Johnson
Joe,Mary,Cathy
40000,50000,44000
WiredTiger supports both storage formats, and can mix and match the storage
of columns within a logical table.
A \em table in WiredTiger consist of one or more \em "column groups" that
together hold all of the columns in primary key order; and zero or more \em
indices that enable fast lookup of records by columns in orders other than
the primary key.
Applications describe the format of their data by supplying a schema to
WT_SESSION::create. This specifies how the application's data can be split
into fields and mapped onto rows and columns.
@section schema_column_types Column types
By default, WiredTiger works as a traditional key/value store, where the
keys and values are raw byte arrays accessed using a WT_ITEM structure.
Key and value types may also be chosen from a list, or composed of multiple
columns with any combination of types. Keys and values may be up to
(4GB - 512B) bytes in size.
See @subpage_single keyvalue for more details on raw key / value items.
@section schema_format_types Format types
WiredTiger's uses format strings similar to those specified in the Python
struct module to describe the types of columns in a table:
http://docs.python.org/library/struct
@hrow{Format, C Type, Java type, Python type, Notes}
@row{\c x, N/A, N/A, N/A, pad byte\, no associated value}
@row{\c b, \c int8_t, \c byte, \c int, signed byte}
@row{\c B, \c uint8_t, \c byte, \c int, unsigned byte}
@row{\c h, \c int16_t, \c short, \c int, signed 16-bit}
@row{\c H, \c uint16_t, \c short, \c int, unsigned 16-bit}
@row{\c i, \c int32_t, \c int, \c int, signed 32-bit}
@row{\c I, \c uint32_t, \c int, \c int, unsigned 32-bit}
@row{\c l, \c int32_t, \c int, \c int, signed 32-bit}
@row{\c L, \c uint32_t, \c int, \c int, unsigned 32-bit}
@row{\c q, \c int64_t, \c long, \c int, signed 64-bit}
@row{\c Q, \c uint64_t, \c long, \c int, unsigned 64-bit}
@row{\c r, \c uint64_t, \c long, \c int, record number}
@row{\c s, \c char[], \c String, \c str, fixed-length string}
@row{\c S, \c char[], \c String, \c str, NUL-terminated string}
@row{\c t, \c uint8_t, \c byte, \c int, fixed-length bit field}
@row{\c u, WT_ITEM *, byte[], \c str,
raw byte array}
The \c 'r' type is used for record number keys in column stores. It is
otherwise identical to the \c 'Q' type.
The \c 'S' type is encoded as a C language string terminated by a
NUL character.
@m_if{java}
Because of this, the associated Java String may not contain the NUL character.
@m_endif
The \c 't' type is used for fixed-length bit field values. If
it is preceded by a size, that indicates the number of bits to store,
between 1 and 8. That number of low-order bits will be stored in the
table. The default is a size of 1 bit: that is, a boolean.
@m_if{c}
C applications must always use a \c uint8_t type (or equivalently,
unsigned char) for calls to WT_CURSOR::set_value, and a
pointer to the same for calls to WT_CURSOR::get_value.
@m_endif
If a bit field
value is combined with other types in a packing format, it is equivalent to
\c 'B', and a full byte is used to store it.
When referenced by a record number (that is, a key format of
'r'), the \c 't' type will be stored in a fixed-length
column-store, and will not have an out-of-band value to indicate the record
does not exist. In this case, a 0 byte value is used to indicate the
record does not exist. This means removing a record with WT_CURSOR::remove
is equivalent to storing a value of 0 in the record with WT_CURSOR::update
(and storing a value of 0 in the record will cause cursor scans to skip the
record). Additionally, creating a record past the end of an object implies
the creation of any missing intermediate records, with byte values of 0.
The \c 'u' type is for raw byte arrays: if it appears at the end of a
format string (including in the default \c "u" format for untyped tables),
the size is not stored explicitly. When \c 'u' appears within a format
string, the size is stored as a 32-bit integer in the same byte order as
the rest of the format string, followed by the data.
There is a default collator that gives lexicographic (byte-wise)
comparisons, and the default encoding is designed so that lexicographic
ordering of encoded keys is usually the expected ordering. For example,
the variable-length encoding of integers is designed so that they have the
natural integer ordering under the default collator.
See @subpage packing for details of WiredTiger's packing format.
WiredTiger can also be extended with custom collators by implementing the
@m_if{c}
WT_COLLATOR interface.
@m_else
WT_COLLATOR interface (C only).
@m_endif
@section schema_key_and_value_formats Key and value formats
Every table has a key format and a value format as describe in
@ref schema_column_types. These types are configured when the table is
created by passing \c key_format and \c value_format keys to
WT_SESSION::create.
For example, a simple row-store table with strings as both keys and
values would be created as follows:
@snippet ex_all.c Create a table
A simple column-store table with strings for values would be created as
follows:
@snippet ex_all.c Create a column-store table
@section schema_cursor_formats Cursor formats
Cursors for a table have the same key format as the table itself.
@m_if{c}
The key columns of a cursor are set with WT_CURSOR::set_key and accessed with
WT_CURSOR::get_key. WT_CURSOR::set_key is analogous to \c printf,
and takes a list of value
@m_else
The key columns of a cursor are set with the \c Cursor.putKey* methods
and accessed with the \c Cursor.getKey* methods. \c Cursor.putKey* methods
must be called
@m_endif
in the order the key columns are configured in \c
key_format.
For example, setting the key for a row-store table with strings as keys
would be done as follows:
@snippet ex_all.c Set the cursor's string key
For example, setting the key for a column-store table would be done as
follows:
@snippet ex_all.c Set the cursor's record number key
A more complex example, setting a composite key for a row-store table where
the key_format was \c "SiH", would be done as follows:
@snippet ex_all.c Set the cursor's composite key
The key's values are accessed with
@m_if{c}
WT_CURSOR::get_key, which is analogous
to \c scanf, and takes a list of pointers to values in the same order:
@m_else
successive calls to \c Cursor.getKey* methods:
@m_endif
@snippet ex_all.c Get the cursor's string key
@snippet ex_all.c Get the cursor's record number key
@snippet ex_all.c Get the cursor's composite key
Cursors for a table have the same value format as the table, unless a
projection is configured with WT_SESSION::open_cursor. See
@ref cursor_projections for more information.
@m_if{c}
WT_CURSOR::set_value is used to set value columns, and
WT_CURSOR::get_value is used to get value columns, in the same way as
described for WT_CURSOR::set_key and WT_CURSOR::get_key.
@m_else
\c The Cursor.putValue* methods are used to set value columns, and
\c Cursor.getValue* are used to get value columns, in the same way as
described for \c Cursor.putKey* and \c Cursor.getKey*.
@m_endif
@section schema_columns Columns
The columns in a table can be assigned names by passing a \c columns key to
WT_SESSION::create. The column names are assigned first to the columns in
the \c key_format, and then to the columns in \c value_format. There must be
a name for every column, and no column names may be repeated.
For example, a column-store table with an employee ID as the key and
three columns (department, salary and first year of employment), might
be created as follows:
@snippet ex_all.c Create a table with columns
In this example, the key's column name is \c id, and the value's column
names are \c department, \c salary, and \c year-started (where \c id maps to
the column format \c r, \c department maps to the column value format \c
S, \c salary maps to the value format \c i and \c year-started maps to the
value format \c H).
Once the table is created, there is no need to call WT_SESSION::create
during subsequent runs of the application. However, it's worthwhile
making the call anyway as it both verifies the table exists and the
table schema matches the schema expected by the application.
@section schema_column_groups Column groups
Once column names are assigned, they can be used to configure column
groups. Column groups are primarily used to define storage in order to
tune cache behavior, as each column group is stored in a separate file.
There are two steps involved in setting up column groups: first, pass a
list of names for the column groups in the \c colgroups configuration
key to WT_SESSION::create. Then make a call to WT_SESSION::create for
each column group, using the URI colgroup:\:\ and a \c columns key in the configuration. Every column
must appear in at least one column group; columns can be listed in
multiple column groups, causing the column to be stored in multiple
files.
For example, consider the following data being stored in a WiredTiger
table:
@snippet ex_schema.c schema declaration
If we primarily wanted to access the population information by itself,
but still wanted population information included when accessing other
information, we might store all of the columns in one file, and store
an additional copy of the population column in another file:
@snippet ex_schema.c Create a table with column groups
Column groups always have the same key as the table. This is particularly
useful for column stores, because record numbers are not stored explicitly
on disk, so there is no repetition of keys across multiple files. Keys
will be replicated in multiple files in the case of row-store column groups.
A cursor can be opened on a column group by passing the column group's
URI to the WT_SESSION::open_cursor method. For example, the population
can be retrieved from both of the column groups we created:
@snippet ex_schema.c Read population from the primary column group
@snippet ex_schema.c Read population from the standalone column group
Key columns may not be included in the list of columns for a column
group. Because column groups always have the same key as the table, key
columns for column groups are retrieved using WT_CURSOR::get_key, not
WT_CURSOR::get_value.
@section schema_indices Indices
Columns are also used to create and configure indices on tables.
Table indices are automatically updated whenever the table is modified.
Table index cursors are read-only and cannot be used for update operations.
To create a table index, call WT_SESSION::create using the URI
index:\:\, listing a column in the
configuration.
Continuing the example, we might open an index on the \c country column:
@snippet ex_schema.c Create an index
Cursors are opened on indices by passing the index's URI to the
WT_SESSION::open_cursor method.
Index cursors use the specified index key columns for
@m_if{c}
WT_CURSOR::get_key and WT_CURSOR::set_key.
@m_else
\c Cursor.getKey* and \c Cursor.putKey* calls.
@m_endif
For example, we can retrieve information from
the \c country index as follows:
@snippet ex_schema.c Search in a simple index
To create an index with a composite key, specify more than one column
to the WT_SESSION::create call:
@snippet ex_schema.c Create an index with a composite key
To retrieve information from a composite index requires a more complicated
@m_if{c}
WT_CURSOR::set_key call,
@m_else
set of \c Cursor.putKey* calls,
@m_endif
but is otherwise the same:
@snippet ex_schema.c Search in a composite index
@section schema_index_immutable Immutable indices
It is possible to create an index with the \c immutable configuration
setting enabled. This setting tells WiredTiger that the index keys for a
record do not change when records are updated. This is an optimization
that it saves a remove and insert into the index whenever a value in the
primary table is updated.
If immutable is configured when updates should alter the content of
the index it is possible to corrupt data.
An example of using an immutable index is:
@snippet ex_schema.c Create an immutable index
@section schema_index_projections Index cursor projections
By default, index cursors return all of the table's value columns from
@m_if{c}
WT_CURSOR::get_value.
@m_else
\c Cursor.getValue* calls.
@m_endif
The application can specify that a subset of the
usual columns should be returned in calls to
@m_if{c}
WT_CURSOR::get_value
@m_else
\c Cursor.getValue
@m_endif
by appending a list of columns to the \c uri parameter of the
WT_SESSION::open_cursor call. This is called a \em projection, see
@ref cursor_projections for more details.
In the case of index cursors, a projection can be used to avoid lookups
in column groups that do not hold columns relevant to the operation.
The following example will return just the table's primary key (a record
number, in this case) from the index:
@snippet ex_schema.c Return the table's record number key using an index
Here is an example of a projection that returns a subset of columns from
the index:
@snippet ex_schema.c Return a subset of the value columns from an index
For performance reasons, it may be desirable to include all columns for
a performance-critical operation in an index, so that it is possible to
perform index-only lookups where no column group from the table is
accessed. In this case, all of the "hot" columns should be included in the
index (always list the "real" index key columns first, so they will
determine the sort order). Then, open a cursor on the index that doesn't
return any value columns, and no column group will be accessed.
@snippet ex_schema.c Access only the index
Index cursors for column-store objects may not be created using the
record number as the index key (there is no use for a secondary index
on a column-store where the index key is the record number).
@section schema_examples Code samples
The code included above was taken from the complete example program
@ex_ref{ex_schema.c}.
Here is another example program, @ex_ref{ex_call_center.c}.
@snippet ex_call_center.c call-center decl
@snippet ex_call_center.c call-center work
*/