On 05/12/2018 14:38, Stephen Frost wrote:
Greetings,
* Chris Withers (chris@xxxxxxxxxxx) wrote:
On 30/11/2018 15:33, Stephen Frost wrote:
* Chris Withers (chris@xxxxxxxxxxx) wrote:
On 28/11/2018 22:49, Stephen Frost wrote:
For this, specifically, it's because you end up with exactly what you
have: a large index with tons of duplicate values. Indexes are
particularly good when you have high-cardinality fields. Now, if you
have a skewed index, where there's one popular value and a few much less
popular ones, then that's where you really want a partial index (as I
suggest earlier) so that queries against the non-popular value(s) is
able to use the index and the index is much smaller.
Interesting! In my head, for some reason, I'd always assumed a btree index
would break down a char field based on the characters within it. Does that
never happen?
Not sure what you mean by 'break down a char field'.
Rather than breaking into three buckets ('NEW', 'ACK', 'RSV'), a more
complicated hierarchy ('N', 'NE', 'A', 'AC', etc).
If I changed this to be an enum field, would != still perform poorly or can
the query optimiser spot that it's an enum and just look for the other
options?
I don't believe we've got any kind of optimization like that today for
enums.
Good to know, I see query optimisers as magic, and postgres often seems
to achieve magic results ;-)
Chris
