postgresql pivot using crosstab

Question

I have trouble using crosstab() in postgresql-11.

Here is my table,

CREATE TABLE monitor(tz timestamptz, level int, table_name text, status text);

The table monitors events on other tables. It contains

table_name (table on which the event occurred)
timestamp(time at which the event occurred)
level (level of the event)
status of the event (start/end of the event)

Here is the sample data to it.

                tz                | level | status |  table_name  
----------------------------------+-------+--------+--------------
 2019-10-24 16:18:34.89435+05:30  |     2 | start  | test_table_2
 2019-10-24 16:18:58.922523+05:30 |     2 | end    | test_table_2
 2019-11-01 10:31:08.948459+05:30 |     3 | start  | test_table_3
 2019-11-01 10:41:22.863529+05:30 |     3 | end    | test_table_3
 2019-11-01 10:51:44.009129+05:30 |     3 | start  | test_table_3
 2019-11-01 12:35:23.280294+05:30 |     3 | end    | test_table_3

Given a timestamp, I want to list out all current events at that time. It could be done using the criteria,

start_time >= 'given_timestamp' and end_time <= 'given_timestamp'

So I tried to use crosstab() to pivot the table over columns table_name,status and timestamp. My query is,

with q1 (table_name, start_time,end_time) as
    (select * from crosstab
    ('select table_name, status, tz from monitor ')
    as finalresult (table_name text, start_time timestamptz, end_time timestamptz)), 
q2 (level,start_time,end_time) as 
    (select * from crosstab('select level, status, tz from monitor ') 
    as finalresult (level int, start_time timestamptz, end_time timestamptz)) 
select q1.table_name,q2.level,q1.start_time,q1.end_time 
    from q1,q2 
    where q1.start_time=q2.start_time;

The output of the query is,

 table_name  | level |            start_time            |             end_time             
--------------+-------+----------------------------------+----------------------------------
 test_table_2 |     2 | 2019-10-24 16:18:34.89435+05:30  | 2019-10-24 16:18:58.922523+05:30
 test_table_3 |     3 | 2019-11-01 10:31:08.948459+05:30 | 2019-11-01 10:41:22.863529+05:30

But my expected output is,

table_name  | level |            start_time            |             end_time             
--------------+-------+----------------------------------+----------------------------------
 test_table_2 |     2 | 2019-10-24 16:18:34.89435+05:30  | 2019-10-24 16:18:58.922523+05:30
 test_table_3 |     3 | 2019-11-01 10:31:08.948459+05:30 | 2019-11-01 10:41:22.863529+05:30
 test_table_3 |     3 | 2019-11-01 10:51:44.009129+05:30 | 2019-11-01 12:35:23.280294+05:30

How do I achieve the expected output? Or is there any better way other than crosstab?

score 2 · Accepted Answer · 2019-11-01 07:30:50Z

2

I would use a self join for this. To keep the rows on the same level and table together you can use a window function to assign numbers to them so they can be distinguished.

with numbered as (
  select tz, level, table_name, status, 
         row_number() over (partition by table_name, status order by tz) as rn
  from monitor
)
select st.table_name, st.level, st.tz as start_time, et.tz as end_time
from numbered as st
  join numbered as et on st.table_name = et.table_name
                     and et.status = 'end'
                     and et.level = st.level
                     and et.rn = st.rn
where st.status = 'start'
order by st.table_name, st.level;

This assumes that there will never be a row with status = 'end' and an earlier timestamp then the corresponding row with status = 'start'

Online example: https://rextester.com/QYJK57764

edited Nov 1, 2019 at 7:30

answered Nov 1, 2019 at 7:10

user330315

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2 Comments

krithikaGopalakrishnan Over a year ago

Self join produces wrong results when there are two consective events of same level for a table. please see the updated sample data

krithikaGopalakrishnan Over a year ago

Worked! Thanks :) But is there a way to do it with crosstab??

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