[Original] PLSQL list of PostgreSQL traversal tables

We often traverse the system tables in the data dictionary to write SHELL scripts or dynamically create data. Here I use PLSQL to demonstrate three methods to traverse a table.

The table structure is as follows,

t_girl=# \d tmp_1;           Unlogged table "public.tmp_1"  Column  |            Type             | Modifiers----------+-----------------------------+----------- id       | integer                     | log_time | timestamp without time zone |

Here I create a custom type to save my function return values.

create type ytt_record as (id int,log_time timestamp without time zone);

Now let's look at the first function. It is also the most stupid way to traverse.

create or replace function sp_test_record1(IN f_id int) returns setof ytt_record as$ytt$declare i int;declare cnt int;declare o_out ytt_record;begin   i := 0;   cnt := 0;   select count(*)   into  cnt from tmp_1 where id > f_id;   while i < cnt   loop       select id,log_time into strict o_out from tmp_1 where id > f_id order by log_time desc limit 1 offset i;     i := i + 1;     return next o_out;   end loop;end;$ytt$ language plpgsql;

It takes about 3 milliseconds to execute the result.

t_girl=# select  * from sp_test_record1(60); id |          log_time         ----+---------------------------- 85 | 2014-01-11 17:52:11.696354 73 | 2014-01-09 17:52:11.696354 77 | 2014-01-04 17:52:11.696354 80 | 2014-01-03 17:52:11.696354 76 | 2014-01-02 17:52:11.696354 65 | 2013-12-31 17:52:11.696354 80 | 2013-12-30 17:52:11.098336 85 | 2013-12-27 17:52:11.098336 97 | 2013-12-26 17:52:11.696354 94 | 2013-12-24 17:52:09.321394(10 rows)Time: 3.338 ms

Now let's look at the second function. This is a relatively optimized function. We use the loop traversal structure that comes with the system.

create or replace function sp_test_record2(IN f_id int) returns setof ytt_record as$ytt$declare o_out ytt_record;begin  for o_out in   select id,log_time  from tmp_1 where id > f_id order by log_time desc  loop     return next o_out;   end loop;end;$ytt$ language plpgsql;

The running result shows that the time is less than 1 ms.

t_girl=# select * from sp_test_record2(60); id |          log_time         ----+---------------------------- 85 | 2014-01-11 17:52:11.696354 73 | 2014-01-09 17:52:11.696354 77 | 2014-01-04 17:52:11.696354 80 | 2014-01-03 17:52:11.696354 76 | 2014-01-02 17:52:11.696354 65 | 2013-12-31 17:52:11.696354 80 | 2013-12-30 17:52:11.098336 85 | 2013-12-27 17:52:11.098336 97 | 2013-12-26 17:52:11.696354 94 | 2013-12-24 17:52:09.321394(10 rows)Time: 0.660 ms

The last function directly returns the result set using return query.

create or replace function sp_test_record3(IN f_id int) returns setof ytt_record as$ytt$begin  return query select id,log_time  from tmp_1 where id > f_id order by log_time desc ;end;$ytt$ language plpgsql;

This result is equivalent to the SELECT statement directly from the table. The response time is similar to that of the second table.

t_girl=# select sp_test_record3(60);          sp_test_record3         ----------------------------------- (85,"2014-01-11 17:52:11.696354") (73,"2014-01-09 17:52:11.696354") (77,"2014-01-04 17:52:11.696354") (80,"2014-01-03 17:52:11.696354") (76,"2014-01-02 17:52:11.696354") (65,"2013-12-31 17:52:11.696354") (80,"2013-12-30 17:52:11.098336") (85,"2013-12-27 17:52:11.098336") (97,"2013-12-26 17:52:11.696354") (94,"2013-12-24 17:52:09.321394")(10 rows)Time: 0.877 mst_girl=#

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