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Why are point hash index not safe in PostgreSQL?
The reduced size allows PostgreSQL to use it’s cache memory (Aka, shared_buffers) more efficiently. As http://www.postgresql.org/docs/9.2/static/sql-createindex.html point Hash index are still not WAL-safe; which means that they are not 100% reliable for crashes (index has to be reconstructed and wrong response could happen on replications).
When to use a hash index in SQL?
It’s better to use a Hash index for text columns that are searched using = operator only. For example a URL column which needs to be indexed for lookups. A Hash index is approximately 30% the size of a B-Tree index for something like a URL.
Why does MySQL not have hash indices on MyISAM or?
The MEMORY engine uses HASH by default indexing algorithm. MySQL did not complain. Bad News !!! I used SHOW INDEXES FROM. It says the index is BTREE. The CREATE INDEX syntax MySQL Page states that only MEMORY and NDB storage engines can accommodate the HASH INDEX.
Which is faster btree or hash in MySQL?
BTree is not that much slower than Hash for single-row lookup. Since BTree provides very efficient range queries, why bother with anything other than BTree. MySQL does a very good job of caching BTree blocks, so a BTree-based query rarely has to do I/O, which is the biggest time consumer in any query.
How is a bitmap heap scan different from an index scan?
The difference is that, rather than visiting every disk page, a bitmap index scan ANDs and ORs applicable indexes together, and only visits the disk pages that it needs to. This is different from an index scan, where the index is visited row by row in order — meaning a disk page may get visited multiple times. Re: the question in your comment…
How does an index scan work in PostgreSQL?
Yep, that’s exactly it. An index scan will go through rows one by one, opening disk pages again and again, as many times as necessary (some will of course stay in memory, but you get the point).