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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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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. The For some data types the rules include a very simple compression mechanism. So even if the original type has some fixed size when it is it may take different number of bytes when encoded as a tuple field it may occupy variable number of bytes.

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.

For any some variable-length type types we can encounter a value with zero length. Again the length is determined by the offset table. Quite naturally To distinguish a zero-length variable-length value translates to a zero-length field in a tuple. This approach is extended to fixed-size types by introducing a notion of default values. We define specific default values for different types (specified in the table below). If a given value is equal to the corresponding default value then this translates to a zero-length field in a tuple.

To sum things up, when a zero-length field is met (by looking in the offset table) we have the following cases:

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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

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• .

The list of supported data types is as follows:

Integer

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Representation

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

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

Thus for 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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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?

hasNullmap = t[0] & 0b100;

How many bytes are occupied by the nullmap?

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:

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How many bytes are occupied by one offset table entry?

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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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