Postgresql dependency search is stalled on large databases

[patrickgoss]

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Issue

Issue description

In general terms, please describe the issue. What command did you run?
dbt run

Results

What happened? What did you expect to happen?
dbt run is stuck by a long running query thats looking for dependency when running on a postgresql database with an excessive number of relations. The query called by the macro postgres__get_relations in postgresql_relations.sql

System information

The output of dbt --version:

installed version: 0.12.1
   latest version: 0.12.1

The operating system you're running on:
MacOS Mojave
Python 3.6.5

Steps to reproduce

In as much detail as possible, please provide steps to reproduce the issue. Sample data that triggers the issue, example models, etc are all very helpful here.
I think this was due to the number of relations we have, we have over 20k relations and almost all of them have at least one dependency. When running this on a smaller postgresql database - ie test with a docker container - I didnt see the issue and dbt run worked as expected.