Chris wrote:
Jonathan Gray wrote:
We’re experiencing a query performance problem related to the planner
and its ability to perform a specific type of merge.
We have created a test case (as attached, or here:
http://www3.streamy.com/postgres/indextest.sql) which involves a
hypothetical customer ordering system, with customers, orders, and
customer groups.
If we want to retrieve a single customers 10 most recent orders,
sorted by date, we can use a double index on (customer,date);
Postgres’s query planner will use the double index with a backwards
index scan on the second indexed column (date).
However, if we want to retrieve a “customer class’s” 10 most recent
orders, sorted by date, we are not able to get Postgres to use double
indexes.
You don't have any indexes on the 'customerclass' table.
Creating a foreign key doesn't create an index, you need to do that
separately.
Try
create index cc_customerid_class on indextest.customerclass(classid,
customerid);
It could also be that since you don't have very much data (10,000) rows
- postgres is ignoring the indexes because it'll be quicker to scan the
tables.
If you bump it up to say 100k rows, what happens?
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