Expand description
The vector itself.
spec/07-execution.md section 7.1 calls this the widest interface in the system, says every
operator depends on it, and says changing it after twenty operators exist is expensive. So it
is written before the first operator rather than after the fifth.
A vector is a type, a length of at most VECTOR_SIZE, a physical form, a validity
representation and some data. Four of the forms are the ones in spec/04-architecture.md
section 4.3: flat, constant, sequence and dictionary. Run length is the fifth and it is the first
of the encoded ones, which arrive one at a time with the kernels that read them rather than all
at once ahead of anything that can use them.
Dictionary and run length are the pair worth understanding together, because they answer
different questions about the same column. A dictionary says which distinct values there are, so
it wins on low cardinality however the rows are ordered. Run length says where the values stop,
so it wins on a clustered column however many distinct values it has. A column can want either
one without wanting the other, and hits has columns of both kinds.
What is not here yet. Buffers are owned. Section 7.1 says a vector borrowed from a buffer
managed page carries a pin, and there is no buffer manager until M2, so there is nothing to pin
and pretending otherwise would be an interface built against an imaginary caller. Nested types
are not stored yet either, for the same reason: a LIST(STRUCT(...)) is offsets plus child
column chunks, and child column chunks are storage.
Structs§
- Packed
- The bits of a packed vector and what they mean, for a kernel that wants to stay in code space.
- Vector
- A type, a length, a validity representation and some data.
Enums§
- Data
- The values of a flat vector, one Rust vector per physical type.
- Form
- Which physical form a vector is in.
Constants§
- PACKED_
WIDTH_ MAX - The widest a packed code is allowed to be.
- PACKING_
PAYS_ AT - How much smaller packing has to be before it is worth the shift and the mask on every read.
- VECTOR_
SIZE - How many values are in a full vector.