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The field location information is represented as a table of field offsets. This table allows to determine both the offset and the length of each field.

To represent NULL values a tuple may contain a nullmap. Nullmap is skipped if there are no actual NULL values in a given tuple.To be more precise the layout of a tuple looks like this:

1. Header;
2. Nullmap (optionally);
3. Offset table;
4. Value area.

Header

It contains only one byte with flags.

Bits 0-1: Size class of the variable-length area.

Bit 2: Set if the nullmap is presentFlag indicating that the size class is not optimal.

Size class encodes the number of bytes used for entries in the offset table. It is encoded like this:

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NOTE: By the way, the last option works fine for C++ and possibly other languages but for Java it will be hard to support, so it might be excluded.

Nullmap

If a schema has nullable columns then the corresponding tuple may contain a nullmap. The nullmap is skipped if there are no actual NULL values in a given tuple instance.

Nullmap is a bitset that contains N bits, occupies (N + 7) / 8 bytes, where `N` is the number of columns in the schema.

In the bitset if a bit is set to `1` then the respective field is NULL.

Offset Table

The number of elements in the table is equal to the number of columns in the schema.

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Each element in the table specifies where the corresponding field ends. Or in In other words, for each given field the table stores the offset where the next field starts. The last element in the table specifies the end of the last field and at the same time the end of the whole value area and consequently the end of the entire tuple.

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The start of the first element is implied and is always equal to zero.

Value area

Contains A value area contains data for each tuple field one after another as described above. Potentially a field can contain any kind of data.  However there are rules for representation of common data types there. These rules are specified in the following section.

Field Values

Data types for which there are no specified rules still may be put into a tuple as a Binary field, but the interpretation in such cases is fully up to a particular application.

Field Encoding

A tuple field is a sequence of bytes. This sequence is interpreted according to the associated data type provided by the tuple schema. There are different encoding rules for different values. For some data types the rules include a very simple compression mechanism. So even if the original type has fixed size it may take different number of bytes when encoded as a tuple field.

The list of supported data types for tuple fields is the same as in IEP-54[1]. Fixed-size values are stored in little-endian byte order. If a value is equal to NULL then it is absent in the value area. This means that in the offset table the corresponding entry is equal to the previous entry. At the same time the corresponding bit in the nullmap is set.
Hence for any

For some variable-length field as we find its length from the offset table we can encounter the case when the length is equal to zero. At this point the value has to be disambiguated in this way:

1. If the corresponding nullmap bit is clear then this is a zero-length (empty) value;
2. If the corresponding nullmap bit is set then this is a NULL value.

This approach is naturally extended to fixed-size fields. But instead of a zero-length value we interpret it as a value of zero. More specifically:

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types we can encounter a value with zero length. To distinguish a zero-length variable-length value from a null value we use a special magic byte that denotes an empty value. That is an empty value is encoded as single-byte sequence – 0x80. In turn if the original variable-length value starts with the magic byte 0x80 byte then this byte is doubled.

The Number and Decimal types are never empty, at least one significant byte is always present. The variable-length types that use the magic byte for encoding are as follows:

• String;
• Binary;
• Bitmask.

The list of supported data types is as follows:

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Integer Representation

All integer values are stored in the little-endian byte order.

For

Integer Field Compression

Uniform use of offset tables for all fields lets us take advantage of variable-length principle for integer fields.

Thus for integer values it is possible to keep only their significant bytes, omitting their high insignificant bytes. That is even if the original data type of a field is INTEGER and should occupy 4 bytes but the actual value is below 128  and above -129  then we can store it just as a single byte and reflect this in the offset table.

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In SQL standard there are no unsigned integers. So their support may be is omitted for now. But should it become needed for some reason then a compressed unsigned integer must be zero-extended on decompression.

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Number and Decimal have identical representation. The Decimal type has a scale parameter but it is stored in the schema rather than in each tuple. Fields of these types are represented as a byte sequence containing the two's-complement binary value. Unlike all the other types here the bytes go in big-endian order. That is the most significant byte is at the very start of the sequence and the least significant byte is at its end.

Duration Representation

A duration field may occupy either 8 or 12 bytes. It consists of a 64-bit signed seconds part optionally followed by a 32-bit nanoseconds part in range from 0 to 999,999,999.

Period Representation

A period field may occupy 3, 6, or 12 bytes. It consists of 3 signed varlen int parts: years, months, days. When every part fits into 8 bits, we write 3 signed 8-bit integers. When every part fits into 16 bits, we write 3 signed 16-bit integers. Otherwise, every part is written as a signed 32-bit integer.

Unlike Date, every part is independent from others and can be in range from Integer.MIN_VALUE to Integer.MAX_VALUE.

Boolean Representation

A single byte containing 1 for the value of true and 0 for the value of false.

Corollary

If the number of fields is N and t is an array that stores a binary tuple we can find the answers for the following:

Does the tuple contain a nullmap?

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How many bytes are occupied by the nullmap?

nullmapBytes = hasNullmap ? (N + 7) / 8 : 0;

How many bytes are occupied by one offset table entry?

offsetEntryBytes = 1 << (t[0] & 0b11);

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What is offset of the value area?

valueBaseOffset = 1 + nullmapBytes + offsetTableBytes;

What is the whole tuple size?

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In order to build a tuple using minimum possible space it is required to learn two things:

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what is the total length of all non-null values

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. After that we can figure out the minimum possible size of the offset table entries.

Thus, generally speaking, building a binary tuple is a two-pass procedure. Sometimes it might be possible to turn this into a single pass (almost) by over-provisioning the allocated storage for the worst case and then fixing it up at the end.

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