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When to use a bitmap scan in PostgreSQL?
The solution to the problem is to use a bitmap scan. The idea behind a bitmap scan is that a single block is only used once during a scan. It can also be very helpful if you want to use more than one index to scan a single table. PostgreSQL will first scan the index and compile those rows / blocks, which are needed at the end of the scan.
How does parallel index scan work in PostgreSQL?
The basic idea is quite similar to parallel heap scans where each worker (including leader whenever possible) will scan a block (all the tuples in a block) and then get the next block that is required to be scanned. The parallelism is implemented at the leaf level of a btree.
What should the parallel index scan size be in GUC?
A new guc min_parallel_index_scan_size has been introduced which indicates the minimum amount of index data that must be scanned in order for a parallel scan to be considered. Users can try changing the value of this parameter to see if the parallel index plan is effective for their queries.
Is there support for parallelism in PostgreSQL 9.6?
In PostgreSQL 9.6, support for the parallel sequential scan was added. A sequential scan is a scan on a table in which a sequence of blocks is evaluated one after the other. This, by its very nature, allows parallelism.
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).
Where are the secondary indexes stored in PostgreSQL?
All indexes in PostgreSQL are secondary indexes, meaning that each index is stored separately from the table’s main data area (which is called the table’s heap in PostgreSQL terminology). This means that in an ordinary index scan, each row retrieval requires fetching data from both the index and the heap.