OOM on EXPLAIN with lots of nodes

Hello!

I found that EXPLAIN command takes very much memory to execute when huge 
unions are used.
For example the following sql
-- begin sql
create table t (a000 int, a001 int, ... a099 int);
explain select * from (    select a001 a from t    union all    select a001 a from t    union all    ... (1000 times)
...   union all    select a001 a from t
 
) _ where a = 1;
-- end sql
took more than 1GB of memory to execute.

Namely converting of the plan to a human-readable form causes excessive 
memory usage, not planning itself.

By varying the parameters and reading source code I determined that
memory usage linearly depends on (plan nodes count)*(overall columns 
count), thus it quadratically depends on number of tables unionized.

To remove this excessive memory usage I propose
to run deparse_context_for_planstate+deparse_expression in a separate 
memory context and free it after a plan node is generated.

Any reasons to treat this idea as bad?

BTW in this case explain execution is also quite long (I got tens of 
seconds). But I have no immediate ideas how to improve it.

Regards,  Alexey Bashtanov