Postgresql – How to pivot a table skipping bad value in Postgres

postgresqlwindow functions

Given a table like:

 label |   x   |   y   |   z 
-------+-------+-------+------
 AAA   | NULL  |   7   |   9
 BBB   | 0     |  10   |   0
 CCC   | 12    | NULL  |   0

create table coords (label text, x integer, y integer, z integer);
insert into coords(label,x,y,z)
values
    ('AAA',NULL,7,9),
    ('BBB',0,10,0),
    ('CCC',12,NULL,0);

I need to insert in another table values by label when they are not NULL or 0s.
So the final result will be:

label | coord | value |
------+-------+-------+
AAA   |   y   |  7    |
AAA   |   z   |  9    |
BBB   |   y   |  10   |
CCC   |   x   |  12   |

I tried with different OVER/PARTITION but probably I am missing something. Any help?

Best Answer

This is really straight forward:

INSERT INTO new_table 
    (label, coord, value)
SELECT label, 'x', x  FROM coords  WHERE x <> 0 
UNION ALL
SELECT label, 'y', y  FROM coords  WHERE y <> 0 
UNION ALL
SELECT label, 'z', z  FROM coords  WHERE z <> 0 ;

or using LATERAL:

INSERT INTO new_table 
    (label, coord, value)
SELECT c.label, v.coord, v.value  
FROM coords AS c
    , LATERAL 
     ( VALUES ('x', x),
              ('y', y),
              ('z', z)
     ) AS v (coord, value)
WHERE v.value <> 0 ;

Related Solutions

Postgresql – Postgres multiple joins slow query, how to store default child record

You write:

Each customer can have multiple sites, but only one should be displayed in this list.

Yet, your query retrieves all rows. That would be a point to optimize. But you also do not define which site is to be picked.

Either way, it does not matter much here. Your EXPLAIN shows only 5026 rows for the site scan (5018 for the customer scan). So hardly any customer actually has more than one site. Did you ANALYZE your tables before running EXPLAIN?

From the numbers I see in your EXPLAIN, indexes will give you nothing for this query. Sequential table scans will be the fastest possible way. Half a second is rather slow for 5000 rows, though. Maybe your database needs some general performance tuning?

Maybe the query itself is faster, but "half a second" includes network transfer? EXPLAIN ANALYZE would tell us more.

If this query is your bottleneck, I would suggest you implement a materialized view.

After you provided more information I find that my diagnosis pretty much holds.

The query itself needs 27 ms. Not much of a problem there. "Half a second" was the kind of misunderstanding I had suspected. The slow part is the network transfer (plus ssh encoding / decoding, possibly rendering). You should only retrieve 100 rows, that would solve most of it, even if it means to execute the whole query every time.

If you go the route with a materialized view like I proposed you could add a serial number without gaps to the table plus index on it - by adding a column row_number() OVER (<your sort citeria here>) AS mv_id.

Then you can query:

SELECT *
FROM   materialized_view
WHERE  mv_id >= 2700
AND    mv_id <  2800;

This will perform very fast. LIMIT / OFFSET cannot compete, that needs to compute the whole table before it can sort and pick 100 rows.

pgAdmin timing

When you execute a query from the query tool, the message pane shows something like:

Total query runtime: 62 ms.

And the status line shows the same time. I quote pgAdmin help about that:

The status line will show how long the last query took to complete. If a dataset was returned, not only the elapsed time for server execution is displayed, but also the time to retrieve the data from the server to the Data Output page.

If you want to see the time on the server you need to use SQL EXPLAIN ANALYZE or the built in Shift + F7keyboard shortcut or Query -> Explain analyze. Then, at the bottom of the explain output you get something like this:

Total runtime: 0.269 ms

Postgresql – Postgres: “Pivot” unioned table based on one column

If it is faster you have to measure for yourself. However, doing it on the db side sends less data across, so I would assume it to be faster.

The pivot itself is fairly simple. I put your query's result in a table to make the example simpler.

SQL Fiddle

PostgreSQL 9.1.9 Schema Setup:

CREATE TABLE your_query
    ("rel_id" int, "timestamp" timestamp, "y" varchar(1))
;

INSERT INTO your_query
    ("rel_id", "timestamp", "y")
VALUES
    (1, '2013-01-01 00:00:00', 'a'),
    (1, '2013-01-02 00:00:00', 'b'),
    (1, '2013-01-03 00:00:00', 'c'),
    (1, '2013-01-04 00:00:00', 'd'),
    (2, '2013-01-01 00:00:00', 'e'),
    (2, '2013-01-04 00:00:00', 'f'),
    (2, '2013-01-06 00:00:00', 'g')
;

First step is to only return one row per date. That is simply done with a group by:

Query 1:

SELECT timestamp
  FROM your_query
 GROUP BY timestamp
 ORDER BY timestamp

Results:

|                      TIMESTAMP |
|--------------------------------|
| January, 01 2013 00:00:00+0000 |
| January, 02 2013 00:00:00+0000 |
| January, 03 2013 00:00:00+0000 |
| January, 04 2013 00:00:00+0000 |
| January, 06 2013 00:00:00+0000 |

Now wee need to pull the "correct" value into each column. For that we combine an aggregate with a case. The case returns null for all rows for which the condition is not met. the aggregate ignores nulls. That leaves the one value we are looking for:

Query 2:

SELECT timestamp,
       MAX(CASE WHEN rel_id = 1 THEN y END ) AS "1",
       MAX(CASE WHEN rel_id = 2 THEN y END ) AS "2"
  FROM your_query
 GROUP BY timestamp
 ORDER BY timestamp