Histograms in PostgreSQL query tool

Assuming you have a table transactions with a balance column:

SELECT  
		WIDTH_BUCKET(balance, 0, 1000, 15) AS "bucket",
		NUMRANGE(min(balance), max(balance), '[]') AS "range",
		COUNT(*) as freq,
		REPEAT('*',(COUNT(*)::float / max(COUNT(*)) over() * 30)::INT) AS "bar"
	FROM transactions
	GROUP BY bucket
	ORDER BY bucket

Notes:

0 is the smallest number you expect
1000 is the largest number you expect
15 is the number of buckets
30 is the length of the longest bar

If you want to see the empty buckets, generate the series and join:

SELECT 
		buckets.bucket,
		range,
		COALESCE(freq, 0) AS "count",
		REPEAT('*',(freq::float / max(freq) over() * 30)::INT) AS "bar"
	FROM 
		(SELECT GENERATE_SERIES::INT AS "bucket" FROM GENERATE_SERIES(0, 21)) AS "buckets"
	LEFT OUTER JOIN
		(SELECT  WIDTH_BUCKET(balance, 0, 2000, 20) AS "bucket",
			NUMRANGE(min(balance), max(balance), '[]') AS "range",
			COUNT(*) as freq
			FROM transactions
			GROUP BY bucket
		) AS histogram
		ON buckets.bucket = histogram.bucket
	ORDER BY buckets.bucket

Note: the GENERATE_SERIES() parameters should be zero and # buckets + 1 if there might be values outside the WIDTH_BUCKET min and max

If you don’t know the min and max in advance:

WITH minmax AS (SELECT MIN(balance), MAX(balance) FROM transactions)
SELECT 
		buckets.bucket,
		range,
		COALESCE(freq, 0) AS "count",
		REPEAT('*',(freq::float / max(freq) over() * 30)::INT) AS "bar"
	FROM 
		(SELECT GENERATE_SERIES::INT AS "bucket" FROM GENERATE_SERIES(1, 15)) AS "buckets"
	LEFT OUTER JOIN
		(SELECT  WIDTH_BUCKET(balance, min, max+1, 15) AS "bucket",
			NUMRANGE(min(balance), max(balance), '[]') AS "range",
			COUNT(*) as freq
			FROM transactions, minmax
			GROUP BY bucket
		) AS histogram
		ON buckets.bucket = histogram.bucket
	ORDER BY buckets.bucket

Note: the +1 for max in WIDTH_BUCKET is because the max value will end up above the bucket range

Source