| Interface defining constants for the binary table result format. Files in
this format consist of a header followed by zero or more records. Data fields
are encoded as specified in the interfaces
DataInput and
DataOutput , except for the encoding of string values. String values
are encoded in UTF-8 and are preceeded by a 32-bit integer specifying the
length in bytes of this UTF-8 encoded string.
The file header is 13 bytes long:
- Bytes 1-4 contain the ASCII codes for the string "BRTR", which stands
for Binary RDF Table Result.
- Bytes 5-8 specify the format version (an integer).
- Byte 9 specifies some flags, specifically 'distinct' and 'ordered'.
- Bytes 10-13 specify the number of columns of the query result that will
follow (an integer).
Following this are the column headers, which are encoded as UTF-8 strings.
There are as many column headers as the number of columns that has been
specified in the header.
Zero or more records follow after the column headers. This can be a mixture
of records describing a result and supporting records. The results table is
described by the result records which are written from left to right, from
top to bottom. Each record starts with a record type marker (a single byte).
The following records are defined in the current format:
- NULL (byte value: 0):
This indicates a NULL value in the table and consists of nothing more than
the record type marker.
- REPEAT (byte value: 1):
This indicates that the next value is identical to the value in the same
column in the previous row. The REPEAT record consists of nothing more than
the record type marker.
- NAMESPACE (byte value: 2):
This is a supporting record that assigns an ID (non-negative integer) to a
namespace. This ID can later be used in in a QNAME record to combine it with
a local name to form a full URI. The record type marker is followed by a
non-negative integer for the ID and an UTF-8 encoded string for the
namespace.
- QNAME (byte value: 3):
This indicates a URI value, the value of which is encoded as a namespace ID
and a local name. The namespace ID is required to be mapped to a namespace in
a previous NAMESPACE record. The record type marker is followed by a
non-negative integer (the namespace ID) and an UTF-8 encoded string for the
local name.
- URI (byte value: 4):
This also indicates a URI value, but one that does not use a namespace ID.
This record type marker is simply followed by an UTF-8 encoded string for the
full URI.
- BNODE (byte value: 5):
This indicates a blank node. The record type marker is followed by an UTF-8
encoded string for the bnode ID.
- PLAIN_LITERAL (byte value: 6):
This indicates a plain literal value. The record type marker is followed by
an UTF-8 encoded string for the literal's label.
- LANG_LITERAL (byte value: 7):
This indicates a literal value with a language attribute. The record type
marker is followed by an UTF-8 encoded string for the literal's label,
followed by an UTF-8 encoded string for the language attribute.
- DATATYPE_LITERAL (byte value: 8):
This indicates a datatyped literal. The record type marker is followed by an
UTF-8 encoded string for the literal's label. Following this label is either
a QNAME or URI record for the literal's datatype.
- ERROR (byte value: 126):
This record indicates a error. The type of error is indicates by the byte
directly following the record type marker: 1 for a malformed query
error, 2 for a query evaluation error. The error type byte is
followed by an UTF-8 string for the error message.
- TABLE_END (byte value: 127):
This is a special record that indicates the end of the results table and
consists of nothing more than the record type marker. Any data following this
record should be ignored.
author:
Arjohn Kampman |