TOAST (The Oversized-Attribute Storage Technique): compression, out-of-line storage, chunking
Simple terms
A row has to fit inside an 8KB page. So how do you store a 1MB jsonb value in a column? You don't, not inline. When a row comes out too big, Postgres first tries compressing the wide columns, and if that gets it under the limit, it stops there.
If not, the value moves out to a hidden companion table, sliced into fixed-size chunks, leaving a small pointer behind where it used to be. Reading it back means fetching those chunks and rebuilding the value, which is why SELECT * over a wide table costs more than you would guess.
You might be asked
A heap tuple has to fit on an 8KB page, yet you can store a 1MB text or jsonb value in a column. Explain how. Walk me through what the 'toaster' does when a row is too big, the order of operations, the difference between the four storage strategies (PLAIN/MAIN/EXTERNAL/EXTENDED), where the bytes actually end up, and how a value is reassembled on read. Then tell me the practical performance traps TOAST creates.
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