MySQL Bugs: #93827: dict_index_has_desc() is not efficient

Bug #93827	dict_index_has_desc() is not efficient
Submitted:	7 Jan 2019 4:45	Modified:	8 Jan 2019 13:35
Reporter:	zhai weixiang (OCA)	Email Updates:
Status:	Verified	Impact on me:	None
Category:	MySQL Server: InnoDB storage engine	Severity:	S5 (Performance)
Version:	8.0.13	OS:	Any
Assigned to:		CPU Architecture:	Any

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Description:
In function dict_index_has_desc, it will check user defined columns on index until it found a desc column. and will check all if there's no descending column. This is not efficient (especailly when change buffer is frequently used) ,  actually we don't need to check it every time because the attribute is only changed when modifying index... 

How to repeat:
Read the code 

Suggested fix:
Add a flag to dict_index_t which indicates if it's descending index, and update it while loading index or altering index.

Hi,

I actually fully agree with your analysis and find that this is a useful performance bug.

Verified as reported.