feat(frontend): use function name as alias in select item

[ZENOTME]

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What's changed and what's your intention?

PLEASE DO NOT LEAVE THIS EMPTY !!!

when use function, Postgres will use function name as column name

test_db=> select sum(id) from t;
 sum
-----

(1 row)

test_db=> select avg(id) from t;
 avg
-----

(1 row)

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

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• Installation and deployment
• Connector (sources & sinks)
• SQL commands, functions, and operators
• RisingWave cluster configuration changes
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Refer to a related PR or issue link (optional)

[BowenXiao1999]

Previously is ?column? ? 🤔

[ZENOTME]

Seems that's because we want user to specify alias in most of time. Maybe this PR should not be merge.

risingwave/src/frontend/src/handler/create_mv.rs

Line 233 in 8b46da1

// Aggregation without alias is forbidden.

[xiangjinwu]

What is the motivation? FYI:

• The full heuristic used by PostgreSQL for generating column names is here frontend: Derive output column names #3812 (comment)
  • For example, v1::bigint is still v1 but 1::bigint is int8. There are 3 levels of priorities.
• We did want user to specify aliases themselves. But as we add support of discovery of column names (e.g. describe mv1;, select * from information_schema.columns where table_name = 'mv1' although still not \d mv1), it is comparably less an issue now if we generate a non-trivial name that user did not specify explicitly. So if we do need to support this and align with PG, let's just raise this to broader audience for agreement.

[ZENOTME]

The motivation for add this is that I encouter a case in supporting create-table-as:
CREATE TABLE integers2 AS SELECT generate_series(0, 100);
In this case integers2 will have a column name 'generate_series' in postgres.
But create-table-as is kind of like create-mv, it's reasonable if we also force user to specify the column name, such as
CREATE TABLE integers2 AS SELECT generate_series(0, 100) tbl(i);
CREATE TABLE integers2(i) AS SELECT generate_series(0, 100);

[xiangjinwu]

It seems the code change in this PR handles the following:

SELECT generate_series(0, 100);

rather than the intended case:

SELECT * FROM generate_series(0, 100);

The latter is TableFactor::TableFunction/Relation::TableFunction rather than Expr::Function