Merge rows (same values) in PostgreSQL based on row difference - postgresql

Question

I have a table my_tbl in my PostgreSQL 9.5 (x64 Windows) database, which contains data as shown below.
grp id low high avg
1 7 292 322 18.8
1 8 322 352 18.8
1 9 352 22 18.8
1 10 22 52 18.8
1 11 52 82 18.8
1 12 82 112 18.8
4 1 97 127 19.0
4 2 127 157 11.4
4 3 157 187 11.4
4 4 187 217 19.6
4 5 217 247 19.6
4 6 247 277 19.6
4 10 7 37 19.5
4 11 37 67 19.5
4 12 67 97 19.5
6 6 182 212 0.0
6 7 212 242 0.0
6 8 242 272 0.0
6 9 272 302 21.4
6 10 302 332 21.4
6 11 332 2 0.0
6 12 2 32 0.0
7 5 275 305 0.0
7 6 305 335 0.0
7 7 335 5 0.0
7 8 5 35 0.0
7 9 35 65 21.2
7 10 65 95 21.2
7 11 95 125 21.2
7 12 125 155 21.2
Now I would like to merge rows in the above data in the following way. For each grp, if the difference between avg values (the preceding and the succeeding ones) is zero (same value), then all such rows should be merged with the low value of the first row to high value of the last row (where merging should stop).
My expected output is:
grp id low high avg
1 {7,8,9,10,11,12} 292 112 18.8
4 {1} 97 127 19.0
4 {2,3} 127 187 11.4
4 {4,5,6} 187 277 19.6
4 {10,11,12} 7 97 19.5
6 {6,7,8} 182 272 0.0
6 {9,10} 272 332 21.4
6 {11,12} 332 32 0.0
7 {5,6,7,8} 275 35 0.0
7 {9,10,11,12} 35 155 21.2
Does someone care to help or suggest on how this could be achieved using SQL/PLPGSQL?

Answer 1

You can use ARRAY_AGG function and FIRST_VALUE function:
SELECT
grp, ARRAY_AGG(id) AS id, low, high, avg
FROM (
SELECT
grp,
id,
FIRST_VALUE(low) OVER (PARTITION BY grp, avg ORDER BY id) AS low,
FIRST_VALUE(high) OVER (PARTITION BY grp, avg ORDER BY id DESC) AS high,
avg
FROM my_tbl ORDER BY id
) t
GROUP BY grp, avg , low, high
ORDER BY grp;
DB Fiddle