Append-only tables in Greenplum

append-only tables
An append-only (AO) table is a storage representation that allows only appending new rows to a table, but does not allow updating or deleting existing rows. This allows for more compact storage on disk because each row does not need to store the MVCC( Unlike traditional database systems which use locks for concurrency control, Greenplum Database (as does PostgreSQL) maintains data consistency by using a multiversion model (multiversion concurrency control or MVCC). This means that while querying a database, each transaction sees a snapshot of data which protects the transaction from viewing inconsistent data that could be caused by (other) concurrent updates on the same data rows. This provides transaction isolation for each database session.MVCC, by eschewing explicit locking methodologies of traditional database systems, minimizes lock contention in order to allow for reasonable performance in multiuser environments. The main advantage to using the MVCC model of concurrency control rather than locking is that in MVCC locks acquired for querying (reading) data do not conflict with locks acquired for writing data, and so reading never blocks writing and writing never blocks reading. ) transaction visibility info. This saves 20 bytes per row. AO tables can also be compressed.