| Lists: | pgsql-bugs |
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| From: | PG Bug reporting form <noreply(at)postgresql(dot)org> |
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| To: | pgsql-bugs(at)lists(dot)postgresql(dot)org |
| Cc: | gentz(at)futuretap(dot)com |
| Subject: | BUG #15679: Partial HASH index takes too much space |
| Date: | 2019-03-08 20:06:45 |
| Message-ID: | 15679-5e1b6086b202047c@postgresql.org |
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| Lists: | pgsql-bugs |
The following bug has been logged on the website:
Bug reference: 15679
Logged by: Ortwin Gentz
Email address: gentz(at)futuretap(dot)com
PostgreSQL version: 10.6
Operating system: Amazon RDS
Description:
I created a partial HASH index for a sparsely populated column:
CREATE INDEX partial_hash ON mytable USING HASH(my_id) WHERE my_ID IS NOT
NULL;
Even though the my_id VARCHAR(255) column is populated (NON NULL) only for a
few hundred records, the index takes 256 MB of space (for a table with > 10m
records). Also, it doesn't make a difference if the index is created as a
partial index ("WHERE my_ID IS NOT NULL") or as a full index.
In contrast to that, a BTREE index differs considerably in space for full
and partial:
CREATE INDEX full_btree ON mytable (my_id); # 543 MB
CREATE INDEX partial_btree ON mytable (my_id) WHERE my_ID IS NOT NULL; # 16
KB
See also this StackExchange conversation where user jjanes considers the
behavior a bug in the hash index code:
https://dba.stackexchange.com/a/231660/25337
| From: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
|---|---|
| To: | gentz(at)futuretap(dot)com, PostgreSQL mailing lists <pgsql-bugs(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: BUG #15679: Partial HASH index takes too much space |
| Date: | 2019-03-11 09:23:32 |
| Message-ID: | CAKJS1f9rg6sxWKg5mbB8P5w_wGbP2FxKDxgnPy20xOW63-4m+w@mail.gmail.com |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Lists: | pgsql-bugs |
On Sat, 9 Mar 2019 at 09:38, PG Bug reporting form
<noreply(at)postgresql(dot)org> wrote:
> I created a partial HASH index for a sparsely populated column:
>
> CREATE INDEX partial_hash ON mytable USING HASH(my_id) WHERE my_ID IS NOT
> NULL;
>
> Even though the my_id VARCHAR(255) column is populated (NON NULL) only for a
> few hundred records, the index takes 256 MB of space (for a table with > 10m
> records). Also, it doesn't make a difference if the index is created as a
> partial index ("WHERE my_ID IS NOT NULL") or as a full index.
>
> In contrast to that, a BTREE index differs considerably in space for full
> and partial:
>
> CREATE INDEX full_btree ON mytable (my_id); # 543 MB
> CREATE INDEX partial_btree ON mytable (my_id) WHERE my_ID IS NOT NULL; # 16
> KB
Thanks for the report. This issue is being discussed over in
/message-id/flat/CAMkU%3D1x0k%2BdRQHDUgp4BjFeSgxyLBBXyKNY5Pt1Yu6YHB0mhKA%40mail.gmail.com
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services