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Postgres MVCC

Index improvements

Index-only scans

• Was not possible in Postgres until recently.

• Still not always possible.

• Does the value still exist in the indexed row?

Index-only scans were enabled with visibility maps (Postgres 9.2).

For a key [k₁, ..., k_n]:

• Search index for relevant keys. For each ([k₁, ... ,k_n], ctid) pair:
  • If visibility map indicates that page(ctid) has not been updated recently, then the components of [k₁, ... ,k_n] are current.
  • Otherwise, read the page and check to see if the indexed row is current.

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