influx query: how to get historical average - influxdb

Question

I am SQL native struggling with flux syntax (philosophy?) once again. Here is what I am trying to do: plot values of a certain measurement as a ratio of their historical average (say over the past month).
Here is as far as I have gotten:
from(bucket: "secret_bucket")
|> range(start: v.timeRangeStart, stop:v.timeRangeStop)
|> filter(fn: (r) => r._measurement == "pg_stat_statements_fw")
|> group(columns: ["query"])
|> aggregateWindow(every: v.windowPeriod, fn: sum)
|> timedMovingAverage(every: 1d, period: 30d)
I believe this produces an average over the past 30 days, for each day window. Now what I don't know how to do is divide the original data by these values in order to get the relative change, i.e. something like value(_time)/tma_value(_time).

Answer 1

Thanks to #Munun, I got the following code working. I made a few changes since my original post to make things work as I needed.
import "date"
t1 = from(bucket: "secret_bucket")
|> range(start: v.timeRangeStart, stop:v.timeRangeStop)
|> filter(fn: (r) => r._measurement == "pg_stat_statements_fw")
|> group(columns: ["query"])
|> aggregateWindow(every: 1h, fn: sum)
|> map(fn: (r) => ({r with window_value: float(v: r._value)}))
t2 = from(bucket: "secret_bucket")
|> range(start: date.sub(from: v.timeRangeStop, d: 45d), stop: v.timeRangeStop)
|> filter(fn: (r) => r._measurement == "pg_stat_statements_fw")
|> mean(column: "_value")
|> group()
|> map(fn: (r) => ({r with avg_value: r._value}))
join(tables: {t1: t1, t2: t2}, on: ["query"])
|> map(fn: (r) => ({r with _value: (r.window_value - r.avg_value)/ r.avg_value * 100.0 }))
|> keep(columns: ["_value", "_time", "query"])

Answer 2

Here are few steps you could try:
re-add _time after the aggregate function so that you can have same number of records as the original one:
|> duplicate(column: "_stop", as: "_time")
calculate the ratio with two data sources via join and map
The final Flux could be:
t1 = from(bucket: "secret_bucket")
|> range(start: v.timeRangeStart, stop:v.timeRangeStop)
|> filter(fn: (r) => r._measurement == "pg_stat_statements_fw")
|> group(columns: ["query"])
|> aggregateWindow(every: v.windowPeriod, fn: sum)
|> timedMovingAverage(every: 1d, period: 30d)
|> duplicate(column: "_stop", as: "_time")
t2 = from(bucket: "secret_bucket")
|> range(start: v.timeRangeStart, stop:v.timeRangeStop)
|> filter(fn: (r) => r._measurement == "pg_stat_statements_fw")
join(tables: {t1: t1, t2: t2}, on: ["hereIsTheTagName"])
|> map(fn: (r) => ({r with _value: r._value_t2 / r._value_t1 * 100.0}))