Add a datediff implementation for postgres

[mjumbewu]

This PR adds a datediff implementation for PostgreSQL, and updates the integration tests to exercise the various types of date parts the implementation covers.

[clrcrl]

Hi, so sorry about the delay here, kicking off the tests now! I think they might actually fail as we have two concurrent test suits running, but I'll re-trigger the tests if they do fail

[clrcrl]

Database Error in model test_datediff (models/cross_db_utils/test_datediff.sql)
  002151 (22023): SQL compilation error: ['DECADE'] is not a valid date/time component for function DATEDIFF.
  compiled SQL at target/run/dbt_utils_integration_tests/cross_db_utils/test_datediff.sql

The tests almost passed!

I'd be happy to not have a decade implementation here, I feel like it would rarely get used. I suspect 'century', 'millennium' will also fail

I'm going to take a look at the code now as well :)

[clrcrl]

Given that Snowflake doesn't support decade and I suspect century and millenium, let's remove them from this implementation and the tests. Other than that, it's LGTM!

[clrcrl]

I've spent far longer than I'd like to admit deciphering this line (and am still left scratching my head). Fortunately, I've tested it pretty thoroughly and I'm convinced it works 👍

[clrcrl]

OK so I found a bit of weirdness. If you do a datediff between:

• First date: Tuesday 4th Feb 2020
• Second date: Monday 3rd Feb 2020 (yes a prior day)
  Then the result is 1, whereas I think the result should be 0.

This is certainly a strange edge case, so I'm not super concerned here. But wanted to flag it just in case. I'm happy to merge it as is though!

[mjumbewu]

Ugh, you're right. Thanks for catching that. Turns out postgres rounds toward 0 in integer division instead of always down (but casting from double to integer rounds down; I expected the opposite behavior).

[clrcrl]

I'm so glad you know what's happening here ha. I truly had no idea!

[philippe-lavoie]

This is my version of the fix

{% macro postgres__datediff_year(first_date, second_date) %}
    DATE_PART('year', ({{second_date}})::timestamp) - DATE_PART('year', ({{first_date}})::timestamp) 
{% endmacro %}

{% macro postgres__datediff_month(first_date, second_date) %}
    {{ postgres__datediff_year(first_date, second_date) }} * 12 + (DATE_PART('month', ({{second_date}})::timestamp) - DATE_PART('month', ({{first_date}})::timestamp))
{% endmacro %}

{% macro postgres__datediff_day(first_date, second_date) %}
    DATE_PART('day', ({{second_date}})::timestamp - ({{first_date}})::timestamp) 
{% endmacro %}

{% macro postgres__datediff_week(first_date, second_date) %}
    TRUNC(DATE_PART('day', ({{second_date}})::timestamp - ({{first_date}})::timestamp)/7) 
{% endmacro %}

{% macro postgres__datediff_hour(first_date, second_date) %}
    {{ postgres__datediff_day(first_date, second_date) }} * 24 + DATE_PART('hour', ({{second_date}})::timestamp - ({{first_date}})::timestamp )  
{% endmacro %}

{% macro postgres__datediff_minute(first_date, second_date) %}
    {{ postgres__datediff_hour(first_date, second_date) }} * 60 + DATE_PART('minute', ({{second_date}})::timestamp - ({{first_date}})::timestamp ) 
{% endmacro %}

{% macro postgres__datediff_second(first_date, second_date) %}
    {{ postgres__datediff_minute(first_date, second_date) }} * 60 + DATE_PART('minute', ({{second_date}})::timestamp - ({{first_date}})::timestamp ) 
{% endmacro %}

{% macro postgres__datediff(first_date, second_date, datepart) %}

    {% if datepart == 'second' %}
        {{  postgres__datediff_second(first_date, second_date)}}
    {% elif datepart == 'minute' %}
        {{  postgres__datediff_minute(first_date, second_date)}}
    {% elif datepart == 'hour' %}
        {{  postgres__datediff_hour(first_date, second_date)}}
    {% elif datepart == 'week' %}
        {{  postgres__datediff_week(first_date, second_date)}}
    {% elif datepart == 'day' %}
        {{  postgres__datediff_day(first_date, second_date)}}
    {% elif datepart == 'month' %}
        {{  postgres__datediff_month(first_date, second_date)}}
    {% elif datepart == 'year' %}
        {{  postgres__datediff_year(first_date, second_date)}}
    {% else %}
        {{ exceptions.raise_compiler_error("macro datediff for PostgreSQL does not support '" + datepart +"'") }}
    {% endif %}

{% endmacro %}

[clrcrl]

I've kicked off the tests but won't have time to review this properly until later this week 😅

[mjumbewu]

@clrcrl No worries. The tests picked up something I missed anyway. Thanks for kicking those off!

[clrcrl]