feat(sql): 'is null' is an alias for '= null' and 'is not null' for '!= null'

[marregui]

MindsDB relies on IS NULL and IS NOT NULL on column literals, e.g.:

create table s( i int);
insert into s values(0), (1), (null);
s where i = null;  -->  s where i IS NULL;
s where i != null; --> s where i IS NOT NULL;

as it rewrites queries to check for null values this way.

mysql> SELECT
    -> m.ts,
    -> d.number_of_rooms, d.location, d.neighborhood, d.days_on_market, m.rental_price,  m FROM questdb.house_rentals_data AS d JOIN mindsdb.home_rentals_model_ts as m
    -> WHERE m.ts > LATEST;
ERROR 1149 (HY000): Execution failed on sql 'SELECT *
FROM house_rentals_data AS d
WHERE d.ts IS NOT NULL ORDER BY d.ts DESC
 LIMIT 10': unexpected token: IS
LINE 3: WHERE d.ts IS NOT NULL ORDER BY d.ts DESC

The approach in this PR is to detect these tokens, IS [NOT] NULL while parsing expressions, wherever they are, and replace them for the equivalent operator token =, '!='.

[puzpuzpuz]

BTW did we miss updating armosx/libquestdb.dylib in one of the previous PRs? I can see it's been updated here.

[bziobrowski]

While it's ok to use = and != on null internally, in SQL it's practically useless and should always evaluate to NULL.
Examples :

1 = null -> NULL
null = 1 -> NULL
null = null NULL
1 != null NULL
null != null NULL

vs

1 is null -> FALSE
1 is not null -> TRUE
null is null -> TRUE
null is not null -> FALSE

[puzpuzpuz]

While it's ok to use = and != on null internally, in SQL it's practically useless and should always evaluate to NULL.

The thing is that in our case this would be a breaking change since many of our users rely on = null/!= null instead of is null/is not null. In our SQL dialect, we don't support three-valued logic.

[marregui]

BTW did we miss updating armosx/libquestdb.dylib in one of the previous PRs? I can see it's been updated here.

I had to build it locally because I was getting a linkage error (UnsatisfiedLink), related to https://questdb-tech.slack.com/archives/C018V7XJKH9/p1646763998810699

[puzpuzpuz]

Great job! Left some comments, mostly nits

[marregui]

thank you very much @puzpuzpuz and @bziobrowski for the thorough review and for all the help, I enjoyed the experience.

What are the rules for merging?, who presses the red button?

[bziobrowski]

I don't have merge privilege so it has to be Andrey .

[puzpuzpuz]

I don't have merge privilege so it has to be Andrey .

Neither do I. 😀

So, it has to be Miguel or someone else from the core team.

[ideoma]

[PR Coverage check]

😍 pass : 60 / 60 (100.00%)

file detail

	path	covered line	new line	coverage
🔵	io/questdb/cutlass/pgwire/PGConnectionContext.java	10	10	100.00%
🔵	io/questdb/griffin/SqlKeywords.java	5	5	100.00%
🔵	io/questdb/griffin/ExpressionParser.java	30	30	100.00%
🔵	io/questdb/griffin/engine/functions/eq/EqDoubleFunctionFactory.java	15	15	100.00%