Optimize postgres query avg by month

we have a report based on avg prices

basically, we filter those prices based on code and date and use UNION for each month

Table example:

date	code	price
2020-01-01	'FS-H21'	150
2020-01-01	'FS-G21'	155
2020-01-02	'FS-H21'	151
2020-01-03	'FS-G21'	148

and the query example:

SELECT period, code, avg(closure)
FROM period_prices
WHERE code = 'FS-F21' AND period BETWEEN '2020-01-01' AND '2020-01-31'
GROUP BY period, code

UNION

SELECT period, code, avg(closure)
FROM period_prices
WHERE code = 'FS-F21' AND period BETWEEN '2020-02-01' AND '2020-02-29'
GROUP BY period, code

...

Reproduced table and query at sample query...

I would like some advice on how can i improve the performance of that query, or if there is a better way to achieve this!

thanks

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