partition pruning only works for select but update

Hi,

  We have a table have range partition (about 5K partitions) , when

Explain select count(*) from table where  partitionkey between  to_timestamp() and to_timestamp();

It show

Aggregate  (cost=15594.72..15594.73 rows=1 width=8)

   ->  Append  (cost=0.15..15582.00 rows=5088 width=0)

         Subplans Removed: 5086

But when

Explain update table set .. where  partitionkey between  to_timestamp() and to_timestamp();

  It still show all of partitions with update …

enable_partition_pruning keep defaut value ‘on’, It’s expected ?  and we found for update sql with same where condition, it consumes huge memory than select.

Database version is Postgres 13.4 on RHEL8.4.

Thanks,

James