B-Tree on Nalar

PostgreSQL B-Tree Deduplication Compresses Duplicate Keys

Wed, 16 Sep 2026 00:00:00 +0700

A PostgreSQL B-tree leaf page can hold many index tuples with identical key values. When deduplication is applicable, PostgreSQL can represent a group of those tuples as one posting-list tuple: the indexed key appears once, followed by a sorted array of heap tuple identifiers.

This representation changes physical index density without changing the logical set of index entries. Each heap tuple remains individually addressable through its TID, while repeated key material occupies less leaf-page space.

PostgreSQL B-Tree Fillfactor Reserves Space Before Page Splits

Tue, 15 Sep 2026 00:00:00 +0700

A PostgreSQL B-tree leaf page has finite space for index tuples. When an incoming tuple belongs on a page that no longer has room, the access method must make space, and a page split can add another leaf page plus a new parent downlink. The fillfactor storage parameter controls how tightly leaf pages are packed at selected points in the index lifecycle, leaving capacity that later writes can consume.

For B-tree indexes, PostgreSQL uses a default fillfactor of 90. A value below 100 deliberately exchanges denser initial storage for free space on leaf pages. That space is not a permanent reservation for a particular row or key. It is simply unused page capacity available to later index activity.

PostgreSQL B-Tree Skip Scan Repositions Index Searches

Tue, 15 Sep 2026 00:00:00 +0700

A multicolumn B-tree does not always require an equality condition on its first column to avoid reading the entire index. PostgreSQL can use skip scan to perform repeated targeted searches when a predicate constrains a later column and the planner estimates that repositioning will bypass enough index entries.

Consider an index whose key order is (region, created_at):

copied = false, 1600)" class="absolute right-3 top-3 z-10 rounded-lg border border-line-strong bg-surface px-2 py-1 font-mono text-[0.65rem] text-muted opacity-0 transition group-hover:opacity-100 hover:bg-ink hover:text-white dark:border-night-line dark:bg-night dark:text-night-muted dark:hover:bg-white dark:hover:text-ink">

CREATE INDEX orders_region_created_at_idx
    ON orders (region, created_at);

A query that filters only created_at has no explicit condition on region:

PostgreSQL B-Tree Deduplication Packs Duplicate Keys

Mon, 14 Sep 2026 00:00:00 +0700

A PostgreSQL B-tree can represent several equal index keys with one physical key value followed by multiple heap tuple identifiers. This representation, called a posting-list tuple, reduces repeated key storage on leaf pages without changing the logical contents of the index.

The mechanism matters most when an indexed value occurs many times. An index on a low-cardinality status column, for example, may contain thousands of entries whose key is pending. Logically those entries still identify separate table tuples. Physically, B-tree deduplication can pack groups of equal keys so the key datum is stored once for a group of TIDs.