Optimizer Trace: plan_prefix should be a comma-separated-list, not array.

Optimizer Trace currently prints plan_prefix as an array:

               "plan_prefix": ["t_outer_1", "t_outer_2", "t_inner_1"],

... sometimes auto-formatter puts each array element onto its own line.

This makes it impossible to use grep.

On the other hand, it doesn't seem to be possible to make any use of the JSON array structure: JSON Path language doesn't provide any convenient way for the most common operation of "find this join prefix".

So, the suggestion is to change the representation to be comma-separated list:

                 "plan_prefix": "t_outer_1,t_outer_2,t_inner_1",