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I have a Fairly large DB containing mostly html documents. I wrote a python script that fetches always 1000 html documents at once from my database cur.execute("SELECT id,url,html_file FROM html ORDER BY id OFFSET %s LIMIT %s;" % (offset, limit)). Afterwards I do a bit of regular expression on the html documents. Because I have a lot html files to go through I track the time how long each step takes. For the first 4000 times retrieving the html documents from the database took around 3 seconds. Now it is up to 4 Minutes. I'm running a Windows 7 machine and took a look at the Resource Monitor. The Database is on its own HDD with nothing else on it. But in the Resource Monitor i could see that the System Process constantly reads something out of my Postgres Folder

Image   PID File    Read (B/sec)    Write (B/sec)   Total (B/sec)   I/O Priority    Response Time (ms)
System  4   E:\PostgreSQL\data\base\10596207\10598404.1 13,855,193  0   13,855,193  Normal  9
System  4   E:\PostgreSQL\data\base\10596207\10598404.1 11,182,442  0   11,182,442  Normal  9

so the question is is this normal or is the system process the culprit here and how do I stop it? (malware and virus scanning are disabled as well as search indexing)

I followed the advice from Reaces and found that superfetch was already disabled. But I also downloaded the process explorer from sysinternals and there I observed something strange. After about the same amount of parsed html documents (about 4,513,000 fetched in package of a 1,000 from the database) the postgres process begins to write a lot of stuff to the HDD up to this point it did read ~3TB from disk and write ~100mb.

But now about 3000 html documents later the process explorer shows me that the postgress proces did write 10GB to the HDD with heavy write access for every cur.execute("SELECT id,url,html_file FROM html ORDER BY id OFFSET %s LIMIT %s;" % (offset, limit)) command

What is the database doing and how do I stop it? my best guess is that the database orders it self to be more efficient the next time but that does not help me now.

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OFFSET is not what you want to use.

The rows skipped by an OFFSET clause still have to be computed inside the server; therefore a large OFFSET might be inefficient.

http://www.postgresql.org/docs/9.3/static/queries-limit.html

Instead, add a where clause along the lines of cur.execute("... WHERE id > %s ORDER BY id LIMIT %s", (last_id_from_previous_batch, 1000)). This will read and return rows starting from that id, instead of scanning OFFSET+LIMIT rows every time and returning only the last chunk. Once offset is large enough, you are likely either swapping, or the query plan starts to require an on-disk sort, causing the I/O.

Also, your code as written is vulnerable to SQL injection. Pass parameterized values as the second argument to execute(), the dbapi module will handle quoting/etc - the % operator will not.

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One of the common disk usage process on a windows 7 machine under the System process is Superfetch. You might want to look into disabling it disabling superfetch.

The reason why it's likely Superfetch is because you're seeing

  • Only reads
  • It's not your OS disk
  • You're running windows 7, not a server version
  • more slowdown as the amount of files increases

If that doesn't help Sysintenals' Process Explorer might help you get a better idea of what's going on. sysinternals

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