create or replace function drop_constraints(_table text) returns text[] language plpgsql as $BODY$
declare
  _rec record;
  _ddl text[];
begin
  for _rec in (
    select con.conname as name,
      'ALTER TABLE ' || _table || ' ADD CONSTRAINT ' || con.conname || ' '
      || pg_get_constraintdef(con.oid) as ddl
    from pg_constraint con
    where con.conrelid=_table::regclass
  ) loop
    _ddl := _ddl || _rec.ddl;
    raise info 'DROP CONSTRAINT %', _rec.name;
    execute('ALTER TABLE ' || _table || ' DROP CONSTRAINT ' || _rec.name);
  end loop;
  return _ddl;
end;
$BODY$;

create or replace function drop_indices(_table text) returns text[] language plpgsql as $BODY$
declare
  _rec record;
  _ddl text[];
begin
  for _rec in (
    select c.relname as name, pg_get_indexdef(i.indexrelid) as ddl
    from pg_index i
      join pg_class c on c.oid=i.indexrelid
    where i.indrelid=_table::regclass
      --excluding pk and unique constraints
      and not exists(select * from pg_depend where objid=i.indexrelid and deptype='i')
  ) loop
    _ddl := _ddl || _rec.ddl;
    raise info 'DROP INDEX %', _rec.name;
    execute('DROP INDEX ' || _rec.name);
  end loop;
  return _ddl;
end;
$BODY$;

create or replace function execute_ddl(_ddl text[]) returns void language plpgsql as $BODY$
declare
  _i int;
begin
  for _i in  1 ..array_upper(_ddl, 1 ) loop
    raise info '%', _ddl[_i];
    execute _ddl[_i];
  end loop;
  return;
end;
$BODY$;

----------------------------------------пример
drop table if exists mytable;
create table mytable (a int primary key, b text unique, c text);
create index mytable_c on mytable(c text_pattern_ops) where c like '8%';

create or replace function update_mytable1() returns void language plpgsql as $BODY$
begin
  insert into mytable select i, i::text, i::text from generate_series( 1 , 1000000 ) as s(i);
end;
$BODY$;

create or replace function update_mytable2() returns void language plpgsql as $BODY$
declare
  _ddl text[];
begin
  _ddl := drop_constraints('mytable') || drop_indices('mytable');
  perform update_mytable1();
  perform execute_ddl(_ddl);
end;
$BODY$;

select update_mytable1(); --1m22s
truncate table mytable;
vacuum full mytable;
select update_mytable2(); --0m36s
\d mytable