Postgres resampling time series data - postgresql

Question

I have OHLCV data of stocks stored in 1-minute increments inside Postgres.
I am trying to resample data to 5 minutes interval. I have used this answer to generate the following SQL query.
Here is the SQL query generated:
SELECT
avg('open') AS open,
avg('high') AS high,
avg('low') AS low,
avg('close') AS close,
avg('volume') AS volume,
avg('open_interest') AS open_interest,
to_timestamp(floor(EXTRACT(epoch FROM 'timestamp') / 300) * 300) AS interval_alias
WHERE 'symbol'='IRFC-N8' GROUP BY interval_alias
I am getting this error:
sqlalchemy.exc.ProgrammingError: (psycopg2.ProgrammingError) function avg(unknown) is not unique
LINE 1: SELECT avg('open') AS open, avg('high') AS high, avg('low') ...
^
HINT: Could not choose a best candidate function. You might need to add explicit type casts.
Could you tell me what went wrong?
Edit 1: Code formatted for better rendering.
Edit 2: According to the answer below, I need to use double quotes around the parameter to the avg function. I am using sqlalchemy to generate the expressions and it is creating single quoted strings. Here is the part of code which generates the avg query:
cols = list()
cols.append(func.avg(self.p.open).label(self.p.open))
cols.append(func.avg(self.p.high).label(self.p.high))
cols.append(func.avg(self.p.low).label(self.p.low))
cols.append(func.avg(self.p.close).label(self.p.close))
cols.append(func.avg(self.p.volume).label(self.p.volume))
cols.append(func.avg(self.p.openinterest).label(self.p.openinterest))
seconds = self._get_seconds()
cols.append(func.to_timestamp(func.floor(func.extract("epoch", "timestamp") / seconds) * seconds).label("interval_alias"))
SqlAlchemy should have known better to use double quotes but its generating single quotes.

Answer 1

db<>fiddle
Your error is the use of single quotes ' instead of double quotes " in your AVG() calls. The single quotes mark texts but you want to name the columns for the average value. So you need the double quotes or can leave them (both variants are shown in the db fiddle).
Edit (The real problem):
It seems that self.p.columnname gives just the column's name, not the column itself. On SQLAlchemy the reference to a specific column is table.c.columnname for referencing the specific column. Please use the p instead of c.
Attention:
If you average all your data you may lose important data as the real minimum and maximum. You may want to aggregate with other functions as MIN or MAX. Maybe the WITHIN GROUP functions could help you.
https://www.postgresql.org/docs/current/static/functions-aggregate.html
https://www.postgresql.org/docs/9.5/static/functions-aggregate.html#FUNCTIONS-ORDEREDSET-TABLE