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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.  🔎Julia Evans 🔍‏ @b0rk 4 Oct 2019
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      sql tips: i heard you like queries so i put more queries in your query (aka "subqueries")pic.twitter.com/IOvbrEZkIk

      29 replies 265 retweets 1,302 likes
    2. David Betz‏ @netfxharmonics 4 Oct 2019
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      Replying to @b0rk

      I've never heard a CTE called a subquery (usually considered to be an improvement over). CTEs have much better performance and predictability than subqueries (which are usually rewritten by the query optimizer as joins).

      2 replies 0 retweets 2 likes
    3. David A Knight‏ @daveaknight 4 Oct 2019
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      Replying to @netfxharmonics @b0rk

      That is going to depend on the database engine. MS SQL Server just expands the CTE out in each place it is used, so behaves just as if it were a sub query. All the CTE ends up doing then is improving readability. The query optimiser will still rewrite.

      1 reply 0 retweets 0 likes
    4. David Betz‏ @netfxharmonics 4 Oct 2019
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      Replying to @daveaknight @b0rk

      Key is "as if it were". I'm still more comfortable with a stronger distinction to avoid the correlated subquery nonsense entirely. Nothing ever becomes a subquery, but things will become join operations. CTE and subquery are ideological abstractions, joins are actual operations.

      2 replies 0 retweets 0 likes
    5.  🔎Julia Evans 🔍‏ @b0rk 4 Oct 2019
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      Replying to @netfxharmonics @daveaknight

      what's a correlated subquery?

      1 reply 0 retweets 0 likes
      RhodiumToad‏ @RhodiumToad 4 Oct 2019
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      Replying to @b0rk @netfxharmonics @daveaknight

      A correlated subquery is one that has outer references, i.e. references to table columns from the enclosing query. Often but not always bad, except in the special case of EXISTS which is better thought of as a kind of join.

      2:48 PM - 4 Oct 2019
      1 reply 0 retweets 0 likes
        1. New conversation
        2. David Betz‏ @netfxharmonics 4 Oct 2019
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          Replying to @RhodiumToad @b0rk @daveaknight

          Once you see how a few of these show up in the execution plan, you just establish a rule to avoid them. Too much rework long term. Just use joins and CTEs.

          1 reply 0 retweets 0 likes
        3. RhodiumToad‏ @RhodiumToad 4 Oct 2019
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          Replying to @netfxharmonics @b0rk @daveaknight

          Used carefully, they can be very effective. (Ditto for lateral joins, which are effectively correlated table subqueries.)

          0 replies 0 retweets 0 likes
        4. End of conversation

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