Re: Index Scans become Seq Scans after VACUUM ANALYSE

Bruce Momjian wrote:
> My second point, that index scan is more risky than sequential scan, is
> outlined above. A sequential scan reads each page once, and uses the
> file system read-ahead code to prefetch the disk buffers. Index scans
> are random, and could easily re-read disk pages to plow through a
> significant portion of the table, and because the reads are random,
> the file system will not prefetch the rows so the index scan will have
> to wait for each non-cache-resident row to come in from disk.

It took a bike ride to think about this one. The supposed advantage of a
sequential read over an random read, in an active multitasking system, is a
myth. If you are executing one query and the system is doing only that query,
you may be right.

Execute a number of queries at the same time, the expected benefit of a
sequential scan goes out the window. The OS will be fetching blocks, more or
less, at random.