Optimizer trace: print join prefix as scalar, not array

Troubleshooting join optimization involves examining what happened whenwe were considering certain join prefix, e.g. t1,t2,t3,...

Optimizer trace prints the join prefix as plan_prefix array:

      "rest_of_plan": [

        {

          "plan_prefix": ["t1", "t2", "t3"],

          "get_costs_for_tables": [

Problems with this representation are:

• it's hard to type all the quotes and spaces when grepping.
• if line length exceeds 80 chars the array is printed as

          "plan_prefix": [

            "t1", 

            "t2", 

            "t3"],

which makes grepp-ing nearly impossible.

A counter-argument is, why doesn't one use JSONPath language to search for an "array of t1, t2, t3?"

Reply to that: MariaDB's JSONPath dialect doesn't support predicates at all. Even if we did support full JSONPath, searching for array of

would look like this

 $.plan_prefix? (@[1]=="t1" && @[2]="t2" && @[3]="t3")

which is still way behind grepping for "plan_prefix": "t1,t2,t3".