expression: support ConstItem() for expression

[wjhuang2016]

What problem does this PR solve?

Add a method for expression.
Sometimes we get a expression, we may want to know if the expression refers to a table.
Presently we have IsCorrelated(), but it's not enough.

For example, See #9751
We want to call evalString, but We cannot eval scalar functions that contain tables.
In the source code of MySQL,

    const uint8 dec = arg_count == 0 ? 0 : args[0]->datetime_precision();
    if (dec > 0) {
      set_data_type_decimal(11 + dec, dec);
    } else {
      set_data_type_longlong();
      max_length = 11;
    }

uint Item::datetime_precision() {
  if (const_item() && result_type() == STRING_RESULT && !is_temporal()) {
    MYSQL_TIME ltime;
    String buf, *tmp;
    MYSQL_TIME_STATUS status;
    DBUG_ASSERT(fixed);
    // Nanosecond rounding is not needed, for performance purposes
    if ((tmp = val_str(&buf)) &&
        !str_to_datetime(tmp, &ltime, TIME_FRAC_TRUNCATE | TIME_FUZZY_DATE,
                         &status))
      return MY_MIN(status.fractional_digits, DATETIME_MAX_DECIMALS);
  }
  return MY_MIN(decimals, DATETIME_MAX_DECIMALS);
}

bool const_item() const { return used_tables() == 0; }

And used_tables() has been used many times in MySQL, I think ReferTable would be helpful for further development.

What is changed and how it works?

Add a method for expression

Check List

Tests

• Unit test

Code changes

• Has exported function/method change
• Has interface methods change

Side effects

Related changes

[eurekaka]

Seems ExtractColumns can provide what you want? CorrelatedColumn should be treated as constant actually?

[wjhuang2016]

Oh, my fault. We need to check also non-deterministic functions and statement parameters

[eurekaka]

Oh from this comment, you need to check if the expression is "purely" constant, i.e, CorrelatedColumn should not be treated as constant because we are not in executor, so ExtractColumns cannot work, sorry for the misleading.

Why would non-deterministic functions matter here? though their return value is non-deterministic, the return type is deterministic?

What does statement parameters mean?

[wjhuang2016]

For non-deterministic functions, their return value is non-deterministic, so you can't use its value to computer the frac part or somethings else.
I believe statement parameters is the parameters used in prepare statement.

[codecov]

Codecov Report

Merging #10004 into master will increase coverage by 0.1013%.
The diff coverage is 92.3076%.

@@               Coverage Diff                @@
##             master     #10004        +/-   ##
================================================
+ Coverage   77.9619%   78.0633%   +0.1014%     
================================================
  Files           405        405                
  Lines         82244      82223        -21     
================================================
+ Hits          64119      64186        +67     
+ Misses        13392      13332        -60     
+ Partials       4733       4705        -28

[qw4990]

LGTM

[alivxxx]

LGTM

[alivxxx]

/run-all-tests