Implement group by index

[kuba--]

Signed-off-by: kuba-- kuba@sourced.tech

Closes: #520, #524, #535

[kuba--]

This is not nice to have solution, but at least it fixes: #524, #520, so I'm totally open for any alternative approaches.

So far I tested against following combination of queries:

SELECT commit_hash AS `commit_hash` FROM commits GROUP BY commit_hash;
SELECT commit_hash AS `commit_hash` FROM commits GROUP BY 1;
SELECT SUBSTRING(`commits`.`commit_hash`, 1, 5) AS `commit_hash` FROM commits GROUP BY commit_hash;
SELECT SUBSTRING(`commits`.`commit_hash`, 1, 5) AS `commit_hash` FROM commits GROUP BY 1;
SELECT count(commit_hash),  repository_id FROM commits GROUP BY 2;

SELECT SUBSTRING(`repositories`.`repository_id`, 1, 1024) AS `repository_id`
FROM `repositories`
  INNER JOIN `files` ON (`repositories`.`repository_id` = SUBSTRING(`files`.`repository_id`, 1, 1024))
GROUP BY 1;

[mcarmonaa]

if weekday == 6 it would return 7, MySQL weekday index range is [0, 6] https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_weekday

[kuba--]

I changed it because tests were failing

[kuba--]

This test is testing a DayOfWeek (not weekday): https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_dayofweek

just expected value was specified by go's weekday, so there is no need for %

[ajnavarro]

@erizocosmico could you have a look?