How ClickHouse Marks Empty Hash Table Cells With Zeroed Memory
Every GROUP BY in ClickHouse is a hash table. The table has to hold millions of keys, be probed once per input row, and be resized while it is being filled — all while the query is …
Memory
How ClickHouse's LowCardinality Column Shares One Dictionary Across Millions of Rows
Every column-oriented database eventually meets the same data: a column of country codes, of status strings, of user agents, of enum-like labels. There are a hundred thousand rows …
How ClickHouse's Variant Column Uses a Discriminator Byte and a Shared Fallback
A Variant column is a tagged union: each row holds a value of one of several types, and the column itself knows the full list of alternatives. Dynamic is the same idea with an …
How PostgreSQL Allocates Memory Fast
Every C program that wants to be fast needs to be fast at allocating memory. PostgreSQL allocates memory in the tightest loop imaginable: a hash join probes its hash table once per …
How tmux Stores Grid Cells in Five Bytes
A terminal grid cell is not small. In the “in-memory” representation it holds a UTF-8 character (up to four bytes), a foreground colour, a background colour, an underline colour, a …