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How to scan an index backwards in PostgreSQL?
The index can also be scanned backward, producing output satisfying ORDER BY x DESC (or more verbosely, ORDER BY x DESC NULLS FIRST, since NULLS FIRST is the default for ORDER BY DESC). …snip…
Why are three indexes used in PostgreSQL?
PostgreSQL starts with index scans on a and b. The result is then joined with c. What’s also important here is that three indexes are used. This happens because of those equality constraints we discussed before. Many people keep asking about explicit versus implicit joins.
Which is the best join strategy in PostgreSQL?
PostgreSQL offers various join strategies. These strategies include hash joins, merge joins, nested loops, and a lot more. We have already shared some of this information in previous posts. More on PostgreSQL join strategies can be found here. Optimizing outer joins ( LEFT JOIN, RIGHT JOIN, etc.) is an important topic.
How does the PostgreSQL query optimizer work and speeds up queries?
There is a variable in the PostgreSQL query optimizer which controls this behavior: The meaning of this parameter is that only up to 8 subselects in the FROM clause will be flattened out. If there are more than 8 subselects, they will be executed without being flattened out.
Can a forward scan of an index produce an output?
This means that a forward scan of an index on a column x produces output satisfying ORDER BY x (or more verbosely, ORDER BY x ASC NULLS LAST). The index can also be scanned backward, producing output satisfying ORDER BY x DESC (or more verbosely, ORDER BY x DESC NULLS FIRST, since NULLS FIRST is the default for ORDER BY DESC).
How does a forward scan of a B-tree index work?
By default, B-tree indexes store their entries in ascending order with nulls last. This means that a forward scan of an index on a column x produces output satisfying ORDER BY x (or more verbosely, ORDER BY x ASC NULLS LAST).