ClickHouse's PODArray Pads 64 Bytes for SIMD and Zeroes the Element at Index -1
Every ClickHouse column is one flat PODArray, padded 64 bytes on both sides.
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ClickHouse's Sparse Column: Only the Rows That Differ, and Offsets Run-Length Encoded as Gaps
A sparse column stores only the rows that differ from the default, plus a sorted offsets array.
ClickHouse's Variant Column: Tag Bytes, Offsets, and a Type Map That Grows at Runtime
A Variant column is a struct of arrays — one sub-column per alternative, one tag byte per row, one offset per row.
ClickHouse Splits GROUP BY into 256 Buckets: Top Hash Bits Route, Low Bits Index
A GROUP BY needs a serial merge at the end — and that’s where the memory high-water mark lives.
ClickHouse Runs a Query as a Graph of Processors: Status Codes and One-Slot Mailboxes
ClickHouse doesn’t call operators in a fixed order. Every processor reports a status — ‘I need data’, ‘my output is full’, ‘call my work()’ — and the executor walks the…