ufk ufk - 9 months ago 82
MySQL Question

is increasing group_concat_max_len to 500000 will cause a performance issue?

I have mysql 5.6 installed.

I have a query that uses group_concat to concatenate a list of user ids into one string with ','. i need to increase the limit of

group_concat_max_len
to 500000.

will that cause a performance issue? why group_concat is limited in the first place ? should I query each line and concat it on server ?

any information regarding the issue would be greatly appreciated.

ufk ufk
Answer Source

the documentation states that The result is truncated to the maximum length that is given by the group_concat_max_len system variable, which has a default value of 1024. The value can be set higher, although the effective maximum length of the return value is constrained by the value of max_allowed_packet.

so increasing group_concat in so many percentage doesn't seem like the right call.. first, it means changing other system variables that my affect the system in different ways

besides that, it's a very big project that has only one group_concat query, so I don't want to modify other system variables only for one specific query.

and in general what @Strawberry said is correct, the value is set to 1024 by default. probably for a good reason.

so I divided my query to two queries..

the first, returns all the data that I need besides the group_concat column

the second, returns all the relevant rows that I used to group_cocat.