PostgreSQL Stores a Row: Headers, Line Pointers, and TOAST
A row is a 23-byte header plus a payload, reached through a 4-byte line pointer on a slotted 8 kB page — the indirection that makes VACUUM, HOT updates, and TOAST possible.
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ClickHouse Sums Floats with Kahan Compensation: The Merge Expression You Must Not Simplify
ClickHouse’s sumKahan carries a compensation term and reconstructs the lost low bits on every add; sum over Decimal widens the accumulator to Decimal128.
ClickHouse's Compressed Column Throws Until You Call decompress(): It Never Caches the Result
ColumnCompressed wraps a column in an LZ4 buffer and throws on every accessor until decompress() is called.
ClickHouse's QBit Column: Float32 Becomes 32 Bit Planes, and Precision Reads Only Some
The QBit column stores a float vector bit-transposed into bit-plane subcolumns, and an analyzer pass makes a distance query read only the planes its precision needs.
ClickHouse's Lazy Column Replication: One Shared Index Per Output Row, Widened on Demand
ColumnReplicated keeps the source column once and stores one row index per output row, so a fan-out JOIN copies an index instead of the data.