feat: add MapColumns method

[molon]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Add MapColumns method: modify the column names in the query results to facilitate align to the corresponding structural fields

User Case Description

I recently used gorm to do aggregation queries based on timescaledb. For example, the following query:

SELECT
    symbol,
    '5min' AS timeframe,
    TRUE AS adj,
    time_bucket('5 minute', t, 'America/New_York') AS bucket,
    time_bucket('5 minute', t, 'America/New_York') AS t,
    FIRST(o, t) AS o,
    MAX(h) AS h,
    MIN(l) AS l,
    LAST(c, t) AS c,
    SUM(v) AS v,
    SUM(n) AS n,
    SUM(v*c)/SUM(v) AS vw
FROM
    market.candle
WHERE
    adj = TRUE AND
    timeframe = '1min' AND 
    symbol = 'AAPL' AND
    time_bucket('5 minute', t, 'America/New_York') > '2023-03-03 17:01:00+08'
GROUP BY
    symbol, bucket
ORDER BY
    symbol, bucket ASC
LIMIT 10000;

bucketWidth := timeFrameToBucketWidth(req.TimeFrame)
db = db.Select(`
	symbol,
	? AS timeframe,
	TRUE AS adj,
	time_bucket(?, t, ?) AS bucket,
	time_bucket(?, t, ?) AS t,
	FIRST(o, t) AS o,
	MAX(h) AS h,
	MIN(l) AS l,
	LAST(c, t) AS c,
	SUM(v) AS v,
	SUM(n) AS n,
	SUM(v*c)/SUM(v) AS vw
`, req.TimeFrame.String(), bucketWidth, s.locationName, bucketWidth, s.locationName)
db = db.Where(`symbol IN ? AND timeframe = '1min' AND adj = ?`, req.Symbols, req.Adjustment)
if !req.Start.IsZero() {
	db = db.Where(fmt.Sprintf("time_bucket(?, t, ?) %s ?", startCompare), bucketWidth, s.locationName, req.Start)
}
if !req.End.IsZero() {
	db = db.Where("time_bucket(?, t, ?) < ?", bucketWidth, s.locationName, req.End)
}
db = db.Group("symbol, bucket")
db = db.Order("symbol ASC, bucket ASC")

The result will be an error ERROR: column "candle.t" must appear in the GROUP BY clause or be used in an aggregate function (SQLSTATE 42803).
However, I cannot put t in Group, which will affect the correct result. So I tried using db = db.Group("symbol, time_bucket('5 minute', t, 'America/New_York')") which unfortunately returns the same error.
So I can only give up returning the t column in the result set, but then I need to customize a nested structure to supplement a bucket column, and then manually assign it to MarketCandle.T. This is very inconvenient.
So I added the MapColumns method. I think this may be a requirement that also exists in other scenarios, especially when GROUP BY is used.

[jinzhu]

Hi @molon

Could you please include some tests to ensure it functions correctly?

[molon]

Hi @molon

Could you please include some tests to ensure it functions correctly?

When I tried to write test cases, I found that my current implementation was unable to meet the scenario where Embedded existed.
Because Embedded will cause duplicate column names to appear.

Do you have any suggestions for this?