alamb commented on code in PR #5015:
URL: https://github.com/apache/arrow-datafusion/pull/5015#discussion_r1083509630


##########
datafusion/core/tests/sqllogictests/README.md:
##########
@@ -46,16 +46,33 @@ cargo test -p datafusion --test sqllogictests -- information
 
 #### Running tests: Postgres compatibility
 
-Test files that start with prefix `pg_compat_` verify compatibility with 
Postgres.
-Datafusion runs these test files during normal sqllogictest runs.
+Test files that start with prefix `pg_compat_` verify compatibility
+with Postgres by running the same script files both with DataFusion and with 
Posgres
 
-In order to run sqllogictests with Postgres execute:
+In order to run the sqllogictests running against a previously running 
Postgres instance, do:
 
 ```shell
-PG_COMPAT=true cargo test -p datafusion --test sqllogictests
+PG_COMPAT=true PG_DSN="postgresql://[email protected]/postgres" cargo test -p 
datafusion --test sqllogictests
 ```
 
-This command requires a docker binary. Check that docker is properly installed 
with `which docker`.
+The environemnt variables:
+
+1. `PG_COMPAT` instructs sqllogictest to run against Postgres (not DataFusion)
+2. `PG_DSN` contains a `libpq` style connection string, whose format is 
described in
+   [the 
docs](https://docs.rs/tokio-postgres/latest/tokio_postgres/config/struct.Config.html#url)
+
+One way to create a suitable a posgres container in docker is to use
+the [Official Image](https://hub.docker.com/_/postgres) with a command
+such as the following. Note the collation **must** be set to `C` otherwise
+`ORDER BY` will not match DataFusion and the tests will diff.
+
+```shell
+docker run \

Review Comment:
   I think `--rm` removes the container. I normally prefer to leave them around 
for debugging 



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