Re: [PERFORM] why we do not create indexes on master - Mailing list pgsql-performance

Possibly some buffer caching is happening, what happens if you then remove the added index and run the query again?

Thank you David,

I used same rationale to convince my colleague — it didn’t work :)

Sort of “pragmatic” person who does what seems working no matter what happens tomorrow.

So I’m seeking for better understanding of what's happening to have other cause to convince him.

Let me break it down once again. The experience is as follows:

- partitioning follows the guide

- master empty, no indexes

- child tables have index on field “field”

- query like

SELECT * FROM “master” WHERE “field” BETWEEN ‘1' AND ‘2’

takes more than 100 sec

- after that my mate adds index on “master”(“field”) — again, all data is in child tables

- same query takes under 1sec

Questions I’d love to clarify:

- Q1: is it correct that described situation happens because index created on master does account data that is already there in child?

- Q2: is it correct that index on master created before inserting record to child tables will not take into account this record?

- Q3: are there any other bad sides of indexes on master table?

Regards,

Val.

On Dec 27 2016, at 19:19, David G. Johnston <david.g.johnston@gmail.com> wrote:

On Tue, Dec 27, 2016 at 8:22 AM, Valerii Valeev <valerii.valeev@mail.ru> wrote:

I have naive idea that it won’t help if index is created before the data is there  — i.e. indexes on master aren’t updated when data loaded to child table.

​Indexes on the master table of a partition scheme never reflect the contents of child​ tables.

In most partitioning schemes the master table is empty so even if it doesn't have an index on a particular field execution would typically be quick.  This is why #4 on the page you linked to:

"""

For each partition, create an index on the key column(s), as well as any other indexes you might want. (The key index is not strictly necessary, but in most scenarios it is helpful. If you intend the key values to be unique then you should always create a unique or primary-key constraint for each partition.)

"""

doesn't say anything about creating other indexes on the master table.  See #1 in that list for an explicit statement of this assumption.

If the master is not empty, and of considerable size, and the field being searched is not indexed, then it is unsurprising that the query would take a long time to execute when obtaining rows from the master table.  If this is the case then you've gotten away from the expected usage of partitions and so need to do things that aren't in the manual to make them work.

David J.

David J.