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Add note for field type
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‎notes/field_type.md

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There are several `types` for each field (column) type, such as `type()`/`real_type()/binlog_type()` ...
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By default, `real_type()` and `binlog_type()` are the same as `type()`:
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```
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// From mysql-8.0/sql/field.h Field
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// ...
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virtual enum_field_types real_type() const { return type(); }
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virtual enum_field_types binlog_type() const {
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/*
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Binlog stores field->type() as type code by default.
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This puts MYSQL_TYPE_STRING in case of CHAR, VARCHAR, SET and ENUM,
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with extra data type details put into metadata.
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We cannot store field->type() in case of temporal types with
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fractional seconds: TIME(n), DATETIME(n) and TIMESTAMP(n),
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because binlog records with MYSQL_TYPE_TIME, MYSQL_TYPE_DATETIME
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type codes do not have metadata.
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So for temporal data types with fractional seconds we'll store
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real_type() type codes instead, i.e.
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MYSQL_TYPE_TIME2, MYSQL_TYPE_DATETIME2, MYSQL_TYPE_TIMESTAMP2,
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and put precision into metatada.
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Note: perhaps binlog should eventually be modified to store
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real_type() instead of type() for all column types.
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*/
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return type();
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}
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// ...
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```
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Here is a list collected from `mysql-8.0/sql/field.h`:
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```
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+------------------------------------------------------------+-------------------------------+-----------------------+------------------------+
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| Field | type() | real_type() | binlog_type() |
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+------------------------------------------------------------+-------------------------------+-----------------------+------------------------+
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| | | | |
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| Field (abstract) | | | |
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| | | | | |
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| +--Field_bit | MYSQL_TYPE_BIT | | |
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| | +--Field_bit_as_char | | | |
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| | | | | |
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| +--Field_num (abstract) | | | |
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| | | +--Field_real (abstract) | | | |
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| | | +--Field_decimal | MYSQL_TYPE_DECIMAL | | |
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| | | +--Field_float | MYSQL_TYPE_FLOAT | | |
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| | | +--Field_double | MYSQL_TYPE_DOUBLE | | |
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| | | | | | |
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| | +--Field_new_decimal | MYSQL_TYPE_NEWDECIMAL | | |
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| | +--Field_short | MYSQL_TYPE_SHORT | | |
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| | +--Field_medium | MYSQL_TYPE_INT24 | | |
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| | +--Field_long | MYSQL_TYPE_LONG | | |
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| | +--Field_longlong | MYSQL_TYPE_LONGLONG | | |
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| | +--Field_tiny | MYSQL_TYPE_TINY | | |
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| | +--Field_year | MYSQL_TYPE_YEAR | | |
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| | | | | |
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| +--Field_str (abstract) | | | |
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| | +--Field_longstr | | | |
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| | | +--Field_string | MYSQL_TYPE_STRING | MYSQL_TYPE_STRING | |
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| | | +--Field_varstring | MYSQL_TYPE_VARCHAR | MYSQL_TYPE_VARCHAR | |
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| | | +--Field_blob | MYSQL_TYPE_BLOB | | |
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| | | +--Field_geom | MYSQL_TYPE_GEOMETRY | | |
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| | | +--Field_json | MYSQL_TYPE_JSON | | |
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| | | +--Field_typed_array | real_type_to_type(m_elt_type) | m_elt_type | MYSQL_TYPE_TYPED_ARRAY |
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| | | | | | |
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| | +--Field_null | MYSQL_TYPE_NULL | | |
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| | +--Field_enum | MYSQL_TYPE_STRING | MYSQL_TYPE_ENUM | |
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| | +--Field_set | | MYSQL_TYPE_SET | |
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| | | | | |
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| +--Field_temporal (abstract) | | | |
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| +--Field_time_common (abstract) | | | |
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| | +--Field_time | MYSQL_TYPE_TIME | | |
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| | +--Field_timef | MYSQL_TYPE_TIME | MYSQL_TYPE_TIME2 | MYSQL_TYPE_TIME2 |
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| | | | | |
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| +--Field_temporal_with_date (abstract) | | | |
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| +--Field_newdate | MYSQL_TYPE_DATE | MYSQL_TYPE_NEWDATE | |
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| +--Field_temporal_with_date_and_time (abstract) | | | |
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| +--Field_timestamp | MYSQL_TYPE_TIMESTAMP | | |
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| +--Field_datetime | MYSQL_TYPE_DATETIME | | |
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| +--Field_temporal_with_date_and_timef (abstract) | | | |
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| +--Field_timestampf | MYSQL_TYPE_TIMESTAMP | MYSQL_TYPE_TIMESTAMP2 | MYSQL_TYPE_TIMESTAMP2 |
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| +--Field_datetimef | MYSQL_TYPE_DATETIME | MYSQL_TYPE_DATETIME2 | MYSQL_TYPE_DATETIME2 |
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+------------------------------------------------------------+-------------------------------+-----------------------+------------------------+
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```

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