Correlation (statistics)
Also called: physical correlation, correlation statistic
Correlation measures how closely the physical order of rows on disk matches the sorted order of a column's values, on a scale from -1 to 1. A value near 1 means the table is almost already in that column's order.
What this means
A score from -1 to 1 for how closely a column's values are already laid out in order on disk. Near 1 means the table is basically stored in that column's sorted order, so reading a range is one smooth sweep instead of jumping all over the disk. It's a big input to whether an index scan looks cheap.
Why it matters operationally
It tells the planner whether an index range scan will read disk pages in near-sequential order (fast) or jump around randomly (slow). High correlation makes index scans much cheaper, which is exactly the order that CLUSTER tries to create.