High key (B-tree)
Also called: page high key, upper bound key
The high key is an upper-bound value stored as the first item on a B-tree page (every page except the rightmost on its level). It promises that every key on the page is less than or equal to it. A search compares its target against the high key to decide whether the key it wants might actually live on the right sibling instead.
What this means
The first item on a B-tree page (every page but the rightmost on its level) is an upper-bound value called the high key: "nothing on this page is bigger than this." A search compares its target against the high key to decide whether the value it's after might actually have moved onto the next page to the right.
Why it matters operationally
The high key is what lets PostgreSQL's B-tree be concurrent: if a split moved your key to the right after you read the parent's pointer, the high key tells you so, and you follow the right-link instead of restarting. No high key, no safe "move right".