57d06518d4
On forums with a large amount of posts when a user had a bookmark in the topic, PostgreSQL was using an inefficient query plan to fetch the first post of the topic. When running this ActiveRecord query:
```
topic.posts.with_deleted.where(post_number: 1).first
```
The following query plan was produced:
```
Limit (cost=0.43..583.49 rows=1 width=891) (actual time=3850.515..3850.515 rows=1 loops=1)
-> Index Scan using posts_pkey on posts (cost=0.43..391231.51 rows=671 width=891) (actual time=3850.514..3850.514
rows=1 loops=1)
Filter: ((topic_id = 160918) AND (post_number = 1))
Rows Removed by Filter: 2274520
Planning time: 0.200 ms
Execution time: 3850.559 ms
(6 rows)
```
The issue here is the combination of ORDER BY and LIMIT causing the ineficcient Index Scan using posts_pkey on posts to be used. When we correct the AR call to this:
```
topic.posts.with_deleted.find_by(post_number: 1)
```
We end up with a query that still has a LIMIT but no ORDER BY, which in turn creates a much more efficient query plan:
```
Limit (cost=0.43..1.44 rows=1 width=891) (actual time=0.033..0.034 rows=1 loops=1)
-> Index Scan using index_posts_on_topic_id_and_post_number on posts (cost=0.43..678.82 rows=671 width=891) (actua
l time=0.033..0.033 rows=1 loops=1)
Index Cond: ((topic_id = 160918) AND (post_number = 1))
Planning time: 0.167 ms
Execution time: 0.072 ms
(5 rows)
```
This query plan uses the correct index, `Index Scan using index_posts_on_topic_id_and_post_number on posts`. Note that this is only a problem on forums with a larger amount of posts; tiny forums would not notice the difference. On large forums a query for a topic that takes 1s without a bookmark can take 8-30 seconds, and even end up with 502 errors from nginx.
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Discourse is the 100% open source discussion platform built for the next decade of the Internet. Use it as a:
- mailing list
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To learn more about the philosophy and goals of the project, visit discourse.org.
Screenshots
Browse lots more notable Discourse instances.
Development
To get your environment setup, follow the community setup guide for your operating system.
- If you're on macOS, try the macOS development guide.
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If you're familiar with how Rails works and are comfortable setting up your own environment, you can also try out the Discourse Advanced Developer Guide, which is aimed primarily at Ubuntu and macOS environments.
Before you get started, ensure you have the following minimum versions: Ruby 2.6+, PostgreSQL 10+, Redis 4.0+. If you're having trouble, please see our TROUBLESHOOTING GUIDE first!
Setting up Discourse
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If you're looking for business class hosting, see discourse.org/buy.
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Discourse is built for the next 10 years of the Internet, so our requirements are high.
Discourse supports the latest, stable releases of all major browsers and platforms:
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|---|---|---|
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| Google Chrome | Android | Android |
| Microsoft Edge | ||
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Built With
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Discourse is 100% free and open source. We encourage and support an active, healthy community that accepts contributions from the public – including you!
Before contributing to Discourse:
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The original Discourse code contributors can be found in AUTHORS.MD. For a complete list of the many individuals that contributed to the design and implementation of Discourse, please refer to the official Discourse blog and GitHub's list of contributors.
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Copyright 2014 - 2020 Civilized Discourse Construction Kit, Inc.
Licensed under the GNU General Public License Version 2.0 (or later); you may not use this work except in compliance with the License. You may obtain a copy of the License in the LICENSE file, or at:
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Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.
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