ufk ufk - 13 days ago 9
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

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.