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Currenly classes Field_time, Field_datetime, Field_timestamp are parent classes for all TIME, DATETIME, TIMESTAMP field implementations, and at the same time are instantiable classes implementing the pre-mysql-5.6 style TIME(0), DATETIME(0), TIMESTAMP(0).
The class hierarchy looks like this:
Field_temporal
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Field_temporal_with_date
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Field_datetime -- instantiable
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Field_datetime_with_dec -- abstract
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Field_datetime_hires -- instantiable
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Field_datetimef -- instantiable
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Field_timestamp -- instantiable
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Field_timestamp_with_dec -- abstract
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Field_timestamp_hires -- instantiable
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Field_timestampf -- instantiable
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Field_time -- instantiable
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Field_time_with_dec -- abstract
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Field_time_hires -- instantiable
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Field_timef -- instantiable
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This hierarchy style is too hard to follow because of multiple overriding, and is error prone because children classes can easily forget to override important methods.
For example, Field_datetimef has to overload the following Field_datetime methods:
enum ha_base_keytype key_type() const override;
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double val_real() override;
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longlong val_int() override;
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String *val_str(String *, String *) override;
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bool send_binary(Protocol *protocol) override;
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int cmp(const uchar *,const uchar *) const override;
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void sort_string(uchar *buff,uint length) override;
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uint32 pack_length() const override;
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bool get_date(MYSQL_TIME *ltime, date_mode_t fuzzydate) override;
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uchar *pack(uchar* to, const uchar *from, uint) override;
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const uchar *unpack(uchar* to, const uchar *from, const uchar *from_end,
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uint) override;
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uint size_of() const override;
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The same style of multiple overriding happens in all instantiable Field_datetime, Field_timestamp, Field_time descendant classes.
MDEV-19906 would have to introduce more multiple overriding and make things harder to follow.
Let's do the following changes:
- Introduce instantiable classes Field_datetime0, Field_timestamp0, Field_time0 implementing pre-mysql-5.6 DATETIME(0), TIMESTAMP(0), TIME(0) respectively.
- Change Field_datetime, Field_timestamp, Field_time to be abstract
Implementation of the methods mentioned above will
- migrate from Field_datetime to Field_datetime0, Field_timestamp to Field_timestamp0, Field_time to Field_time0.
- become zero pointers in the abstract classes Field_datetime, Field_timestamp, Field_time, so it won't be possible to leave these methods unimplemented in the instrantiable descendants
The new hierarchy will look like this:
Field_temporal
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Field_temporal_with_date
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Field_datetime -- abstract
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Field_datetime0 -- instantiable
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Field_datetime_with_dec -- abstract
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Field_datetime_hires -- instantiable
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Field_datetimef -- instantiable
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Field_timestamp -- abstract
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Field_timestamp0 -- instantiable
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Field_timestamp_with_dec -- abstract
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Field_timestamp_hires -- instantiable
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Field_timestampf -- instantiable
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Field_time -- abstract
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Field_time0 -- instantiable
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Field_time_with_dec -- abstract
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Field_time_hires -- instantiable
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Field_timef -- instantiable
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