Kashmira Patel wrote:
> I did do an EXPLAIN ANALYZE as well, it also showed a
> sequential scan. The table has about 600+ rows, with around 6 of them
> matching the given id. Wouldn't an index scan be faster in this case?
Not necessarily. It's entirely possible, if your rows are small, that 600 rows will fit on a single disk page. The
indexwill be stored on a(t least one) separate disk page. The cost of loading a page from disk pretty much swamps the
costof processing rows on a page, so in general the server tries to minimize the number of pages used. To use an index
fora one-page table, it'd have to load two pages (the table and the index); to do a sequential scan over a one-page
tableit only has to load the table.
Indexes are useful because they allow the DB to reduce the total number of pages loaded to complete a query.
-Owen
