PERF: reduce SQL queries by aggreating records

[erickguan]

The PR is a huge hack for performance but it works (somehow). Loading records for individual topics takes a huge amount of time in total. It's better to load them once.

I removed first_post_id to previewed_post_id. I don't understand the logic in the serializer but I think you are limiting the field when the post is loaded. It would be better to use another custom_fields to store the post_id so they can be loaded together.

There are many post action related methods in the serializer. It might be better to check the topic status in the client side to decide whether showing the button. Pushing permission check when the user click the button would save us from pushing can_like? to the client.

[angusmcleod]

@fantasticfears Thanks for taking a shot at this. I was thinking along similar lines, but didn't know where to start.

However I'm getting this error when running this branch locally.

It seems that the previewed_post attribute is not getting assigned for existing topics here. Did you have to drop your local DB for this to work? We need to figure out an alternate solution if that is the case as there are a number of existing users (assuming it's still not possible to migrate DBs from a plugin)

[erickguan]

Fixed! I was only testing on a given backup. On that backup, it doesn't show a problem. But I don't know there can be a topic without a previewed_post. It's loaded from either the first post or post as the accepted answer.

[angusmcleod]

@fantasticfears hm I'm still seeing the same issue in a different place now. Could you try checking out master, create some topics, then check out this branch and see if you get the same error? I'm not sure it's because there are topics on my local with no posts in them. I'm super busy at work right now, otherwise I'd dig into a bit more.

[erickguan]

No problem. I'll try to dig different scenarios.

[erickguan]

I've updated the pull request and it's good in my end. I've removed some SQLs which require postgresql 9.4+.

[angusmcleod]

@fantasticfears Thanks! I've now got it working on my local. Perhaps it was the postgres version, albeit I seem to be running 9.6.1.

Merging this now.