Execution plan of query inside pl/pgsql function

Briefly, we have a pl/pgsql function which collects some intermediate data in several temporary tables (“on commit drop”) and then returns REFCURSOR which joins those temporary tables with some static tables.
We`ve noticed that after several executions and fetches of returned cursor (mostly after 4 executions) subsequent executions(more correctly, fetches of returned cusor) become time consuming.
Test case looks like:

begin;
select t.v_resultset from dbo.Get_Object_Fields (v_cObj:='DOCUMENTS_0401002') t;
fetch all from "<unnamed portal 1>";
commit;

First suspicion is inefficiency of execution plan of resulting query.

Output of pg_stat_statement:

1)query with max(total_time) of 4th and subsequent execution

--------------------------------------------------------------------------------
userid              | 376846
dbid                | 892226
query               | select coalesce(so.code, t.obj_code) as obj_code
                |             ,c.code as class_name
                |             ,sp.code as parent_object
                |             ,EDITABLE,ASPTYPE,TITLE,VISIBLE,FIELD_DATA,ATTRIBUTES,FIELD_ALIAS,FIELD_KEY,COPY_METHOD,NASTR_SHOW_DISABLE,LEN_VALUE,ORDERS,FIELD_CALCULATION,INPUT_MASK
                |         from (
                |                select
                |                       object as OBJECT
                |                      ,obj_code as OBJ_CODE
                |                      ,max(parent_object) as PARENT_OBJECT
                |                      ,max(case when p.prop_code='EDITABLE' then substr(v.value,1,4000) end) as EDITABLE
                |                      ,max(case when p.prop_code='ASPTYPE' then substr(v.value,1,4000) end) as ASPTYPE
                |                      ,max(case when p.prop_code='TITLE' then substr(v.value,1,4000) end) as TITLE
                |                      ,max(case when p.prop_code='VISIBLE' then substr(v.value,1,4000) end) as VISIBLE
                |                      ,coalesce(nullif(max(case when p.prop_code='CONTROLSOURCE' then substr(v.value,1,4000) end),''),
                |
calls               | 1
total_time          | 3681.509
rows                | 37
shared_blks_hit     | 2759046
shared_blks_read    | 0
shared_blks_dirtied | 0
shared_blks_written | 0
local_blks_hit      | 19
local_blks_read     | 0
local_blks_dirtied  | 0
local_blks_written  | 0
temp_blks_read      | 62
temp_blks_written   | 60
blk_read_time       | 0
blk_write_time      | 0

2) corresponding query during first fast executions***

--------------------------------------------------------------------------------
userid              | 376846
dbid                | 892226
query               | select coalesce(so.code, t.obj_code) as obj_code
                |             ,c.code as class_name
                |             ,sp.code as parent_object
                |             ,EDITABLE,ASPTYPE,TITLE,VISIBLE,FIELD_DATA,ATTRIBUTES,FIELD_ALIAS,FIELD_KEY,COPY_METHOD,NASTR_SHOW_DISABLE,LEN_VALUE,ORDERS,FIELD_CALCULATION,INPUT_MASK
                |         from (
                |                select
                |                       object as OBJECT
                |                      ,obj_code as OBJ_CODE
                |                      ,max(parent_object) as PARENT_OBJECT
                |                      ,max(case when p.prop_code='EDITABLE' then substr(v.value,1,4000) end) as EDITABLE
                |                      ,max(case when p.prop_code='ASPTYPE' then substr(v.value,1,4000) end) as ASPTYPE
                |                      ,max(case when p.prop_code='TITLE' then substr(v.value,1,4000) end) as TITLE
                |                      ,max(case when p.prop_code='VISIBLE' then substr(v.value,1,4000) end) as VISIBLE
                |                      ,coalesce(nullif(max(case when p.prop_code='CONTROLSOURCE' then substr(v.value,1,4000) end),''),
                |
calls               | 1
total_time          | 23.303
rows                | 37
shared_blks_hit     | 5154
shared_blks_read    | 0
shared_blks_dirtied | 0
shared_blks_written | 0
local_blks_hit      | 19
local_blks_read     | 0
local_blks_dirtied  | 0
local_blks_written  | 0
temp_blks_read      | 0
temp_blks_written   | 0
blk_read_time       | 0
blk_write_time      | 0

But I can`t get why plan has been changing after 4 executions?
So the questions are:

1. why execution of a query becomes slow after 4th execution of stored function?
2. how to get the real execution plan of queries inside pl/pgsql functions?

Thank you.