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RhodiumToad's profile
RhodiumToad
RhodiumToad
RhodiumToad
@RhodiumToad

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RhodiumToad

@RhodiumToad

Find me on IRC: #postgresql or #freebsd on http://freenode.net . (he/him)

Joined February 2019

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    1. Dave Cramer‏ @dave_cramer 15 Sep 2019
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      Replying to @pg_xocolatl @l_avrot and

      I suspect "slightly" really means infinitesimally faster as the real time is in scanning the table and checking for visibility

      1 reply 0 retweets 0 likes
    2. Vik Fearing‏ @pg_xocolatl 15 Sep 2019
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      Replying to @dave_cramer @l_avrot and

      I have not benchmarked it but I assume that would be true. It's a quick test. 🙂

      1 reply 0 retweets 0 likes
    3. Dave Cramer‏ @dave_cramer 15 Sep 2019
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      Replying to @pg_xocolatl @l_avrot and

      I'd be interested to know if you could measure it, I suspect it is less than the noise

      2 replies 0 retweets 2 likes
    4. Vik Fearing‏ @pg_xocolatl 15 Sep 2019
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      Replying to @dave_cramer @l_avrot and

      Okay, are you ready? The difference between count(*) and count(1) is a spin through this: for (i = 1; i <= numTransInputs; i++) { if (fcinfo->args[i].isnull) return; } But for each row, so it's probably like tens of microseconds on a large grouping!

      1 reply 0 retweets 9 likes
    5. Lukas Eder‏ @lukaseder 19 Sep 2019
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      Replying to @pg_xocolatl @dave_cramer and

      okidoki, I benchmarked this, counting 1M rows many times. TL;DR: - MySQL: No difference - Oracle: No difference - PostgreSQL: Significant difference (surprisingly significant!) - SQL Server: No difference Details here:https://blog.jooq.org/2019/09/19/whats-faster-count-or-count1/ …

      3 replies 10 retweets 20 likes
    6. Vik Fearing‏ @pg_xocolatl 19 Sep 2019
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      Replying to @lukaseder @dave_cramer and

      Wow! 😱

      1 reply 0 retweets 1 like
    7. Lætitia‏ @l_avrot 19 Sep 2019
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      Replying to @pg_xocolatl @lukaseder and

      New patch on the way?

      1 reply 0 retweets 2 likes
    8. Lukas Eder‏ @lukaseder 19 Sep 2019
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      Replying to @l_avrot @pg_xocolatl and

      Well, I'd love to hear why it's so significant. Am I doing something substantially wrong in my benchmark? Can you folks reproduce it? Code here:https://gist.github.com/lukaseder/2611212b23ba40d5f828c69b79214a0e …

      2 replies 0 retweets 2 likes
    9. RhodiumToad‏ @RhodiumToad 19 Sep 2019
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      Replying to @lukaseder @l_avrot and

      I've tested this on occasion in the past. The problem is that the relative performance in benchmarks will depend wholly on the fraction of the time attributable to count(), so it's never the same between different queries.

      1 reply 0 retweets 0 likes
    10. RhodiumToad‏ @RhodiumToad 19 Sep 2019
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      Replying to @RhodiumToad @lukaseder and

      Picking a single query and measuring the overhead specific to count() (this isn't simple) suggests that in fact, count(1) is 50% slower than count(*): on the machine I tested on, this is roughly 4 ns for count(*) and 6 ns for count(1)

      1 reply 0 retweets 0 likes
      RhodiumToad‏ @RhodiumToad 19 Sep 2019
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      Replying to @RhodiumToad @lukaseder and

      But the rest of the query usually drowns that out; e.g. in my tests that represented a difference of 2 seconds for a query taking 47 seconds

      4:14 AM - 19 Sep 2019
      1 reply 0 retweets 0 likes
        1. New conversation
        2. RhodiumToad‏ @RhodiumToad 19 Sep 2019
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          Replying to @RhodiumToad @lukaseder and

          For example, consider the performance difference between "select count(1) from ...", "select count(1), count(2) from ...", and "select count(1), count(*) from ..." on the same input. (Note that the two count() calls MUST be different or they will be combined)

          1 reply 0 retweets 0 likes
        3. Lukas Eder‏ @lukaseder 19 Sep 2019
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          Replying to @RhodiumToad @l_avrot and

          2/47 seconds is still 5%, though. Does the parser have some pre-existing way of recognising constant expressions like "1"? Might be a low hanging fruit... It probably isn't, as these things are often more complicated than it seems from the outside.

          2 replies 0 retweets 0 likes
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