Also called: cached column offset, attribute offset cache
In plain English
attcacheoff is a cached byte offset for a column within a row. If every column before it is fixed-width and NOT NULL, that column always sits at the same offset, so PostgreSQL stores it once and jumps straight there — O(1) access during deforming, no need to walk the earlier columns.
Why it matters
The shortcut works only up to the first variable-length column or the first NULL; after that, offsets depend on the actual data and must be computed by sequential deforming. This is the mechanism behind a practical rule: keep frequently-read, fixed-width columns early in the table so they stay cheap to reach.