PostgreSQL – How to Select Data in the Same Table on Different Rows That Are Empty

countnullpostgresqlselectsqlite

I use this select on my SQLite database:

SELECT attributeX
 FROM table
 GROUP BY attributeX
 HAVING COUNT(attributeY) = 0;

My table:

  attributeX (type integer) |  attributeY (type text)
----------------+-------
  1452 | aaa            
  1452 | bbb
  1452 | 
  1452 | bbb
  1453 |
  1453 |   
  1453 |
  1454 | eee
  1454 | fff
  1455 | iii
      ....

I get result 1453 only when all items in column attributeY are empty for 1453. So in SQLite I get result 1453.

I have the same database only with myschema in PostgreSQL. I use this SELECT:

SELECT attributeX
 FROM myschema.table
 GROUP BY attributeX
 HAVING COUNT(attributeY) = 0;

and I get:

 attributeX
------------
(0 rows)

How to select in PostgreSQL?

SQLite schema of this table:

CREATE TABLE table (
 attribute1 INTEGER PRIMARY KEY,
 attributeX INTEGER NOT NULL,
 attribute2 TEXT NOT NULL,
 attributeY TEXT
);

CREATE INDEX table_attributeY_idx ON table (attributeY);
CREATE INDEX table_attribute2_idx ON table (attribute2);
CREATE INDEX table_attributeX_idx ON table (attributeX);

PostgreSQL schema of this table:

Table "myschema.table"
        Column  |          Type         |                                    Modifier                             | Storage  | Desc
----------------+-----------------------+-------------------------------------------------------------------------+----------+-------
 attribute1     | integer               | not null implicitly nextval('myschema.table_attribute1_seq'::regclass)  | plain    |
 attributeX     | integer               | not null                                                                | plain    |
 attribute2     | character varying(32) | not null                                                                | extended |
 attributeY     | text                  |                                                                         | extended |
Indexy:
    "table_attribute1_pkey" PRIMARY KEY, btree (attribute1)
    "fki_table_attributeX_fkey" btree (attributeX)
Conditions for foreign key:
    "table_attributeX_fkey" FOREIGN KEY (attributeX) REFERENCES myschema.table2(attributeX2) ON UPDATE RESTRICT ON DELETE CASCADE
Has OID: no

Insert:

INSERT INTO myschema.table (attributeX, attribute2, attributeY)
VALUES (%s, %s,%s)... (python script)

Best Answer

You explain:

I get result 1453 only when all items in column attributeY are empty for 1453.

But that's incorrect. Bold emphasis mine. The aggregate function count returns (per documentation):

number of input rows for which the value of expression is not null

The same is true for SQLite (per documentation):

The count(X) function returns a count of the number of times that X is not NULL in a group.

You obviously have one or more rows with a non-null value in attributeY in your Postgres table - probably an empty string ''.

Test with:

SELECT *
FROM   myschema.table
WHERE  attributeX = 1453
AND    attributeY IS NOT NULL;

Be sure to understand the difference between "empty" ('') and NULL:

Empty strings are character types (typically text, varchar or char) with 0 characters ('') - so basically "nothing", the equivalent of 0 for a numeric data type. NULL is possible for any data type and means "unknown". Some clients have a hard time making the difference clear in their display.

Alternative query

To find attributeX where all attributeY are either empty or NULL use this alternative query:

SELECT attributeX
FROM   myschema.table
GROUP  BY attributeX
HAVING NOT bool_or(attributeY <> '');

The expression attributeY <> '' is only true for non-null, non-empty attributeY. The aggregate function bool_or returns (per documentation):

true if at least one input value is true, otherwise false

Those are the results we exclude (NOT ...) and return the rest. Voilá.

There are many other (less elegant) ways to achieve the same. Like:

... HAVING count(CASE WHEN attributeY <> '' THEN 1 END) = 0;
... HAVING count(attributeY <> '' OR NULL) = 0;
... HAVING count(NULLIF(attributeY, '') = 0;

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 – Improve performance on concurrent UPDATEs for a timestamp column in Postgres

Would a different kind of column be faster? For example an integer

No. timestamp and timestamptz are just unsigned 64-bit integers internally anyway.

Is there some way to not lock the column?

It doesn't lock the column. It takes weak table lock that doesn't really block anything except DDL, and takes a row level lock on the row you're updating.

There is no way to prevent the row level lock. It exists because without it behaviour and ordering concurrent updates would be undefined. We don't like undefined behaviour in RDBMSs.

It only blocks concurrent updates of the same row anyway.

Any other tips to improve this?

Not with the detail provided. There's likely a better way to do what you're trying to do, but it'll probably involve taking a few steps back and looking for a different strategy for solving the underlying problem.

In the specific case of cache invalidation I think you might want to look into LISTEN and NOTIFY. Again though, there just isn't enough info here to go on.

Best Answer

Alternative query

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