Core conceptsintermediate
TOAST
The Oversized-Attribute Storage Technique that transparently compresses and/or moves large column values into a side table so main rows stay within a page.
What this means
A row is meant to fit inside one 8 KB page, so what happens when a column holds a whole paragraph or a megabyte of JSON? Postgres quietly compresses the oversized value, and if it's still too big, moves it into a side table and leaves a small pointer in the row. You never see any of this happen, that's the "transparent" part.
Why it matters operationally
TOAST explains why wide text/JSON columns behave differently for storage, compression, and I/O than small scalar columns.
Related errors
Where it lives in the source
src/backend/access/common/toast_internals.c