[MDEV-8862] Wrong field type for MAX(COALESCE(datetime_column)) - Jira

If I start mysql --column-type-info and run this script:

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a DATETIME);

INSERT INTO t1 VALUES ('2001-01-01 10:20:30'),('2001-01-02 10:20:30');

SELECT MAX(a), MAX(COALESCE(a)) FROM t1;

it returns a correct result set:

+---------------------+---------------------+

| MAX(a)              | MAX(COALESCE(a))    |

+---------------------+---------------------+

| 2001-01-02 10:20:30 | 2001-01-02 10:20:30 |

+---------------------+---------------------+

but the metadata is wrong:

Field   1:  `MAX(a)`

Catalog:    `def`

Database:   ``

Table:      ``

Org_table:  ``

Type:       DATETIME

Collation:  binary (63)

Length:     19

Max_length: 19

Decimals:   0

Flags:      BINARY

Field   2:  `MAX(COALESCE(a))`

Catalog:    `def`

Database:   ``

Table:      ``

Org_table:  ``

Type:       VAR_STRING

Collation:  utf8_general_ci (33)

Length:     57

Max_length: 19

Decimals:   0

Flags:

Notice, metadata for MAX(COALESCE(a)) says it's a VAR_STRING column.
The correct result is to return the DATETIME data type, as for the first column.

If I change the column data type from DATETIME to TIME:

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a TIME);

INSERT INTO t1 VALUES ('10:20:30'),('10:20:30');

SELECT MAX(a), MAX(COALESCE(a)) FROM t1;

it also returns VAR_STRING for the second column.

duplicates

MDEV-8910 Wrong metadata or field type for MAX(COALESCE(string_field))

relates to

MDEV-8863 Wrong result for MAX(COALESCE(time_column))

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