Struct DictColumn 

pub struct DictColumn {
    label: &'static str,
    b: StringDictionaryBuilder<UInt16Type>,
    n: usize,
}

A Dictionary<UInt16, Utf8> column that knows how full it is.

StringDictionaryBuilder does not expose its cardinality and returns the overflow as an error from append, which is too late: the seal decision has to be made before the append, because a builder cannot be rolled back. Every enumerable string column in the engine — severity text, span name, event name, metric name and unit — needs exactly this, so it is one type.

Fields

label: &'static str

b: StringDictionaryBuilder<UInt16Type>

n: usize

Implementations

impl DictColumn

pub fn new(label: &'static str) -> Self

pub fn has_headroom(&self, n: usize) -> bool

Whether n more distinct values fit. Callers pass the total number of values they are about to append, because deduplicating them first would cost more than the occasional block sealed a little early.

pub fn append(&mut self, v: &str) -> Result<()>

Empty becomes null rather than a dictionary entry. proto3 cannot distinguish an unset string from an empty one, so every span without a trace_state would otherwise burn a slot and a validity bit to say so.

pub fn finish(&self) -> ArrayRef

Materialise the column without consuming the builder.

finish_cloned, not finish, throughout the seal path: the same code serves both the real seal — whose caller replaces the whole builder afterwards anyway — and the open-block snapshot the read path queries (section 4), which must leave the builder accumulating. The cost is one buffer copy per column instead of a move, which at the 32 MiB block target is a few milliseconds once per block.