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SQLite training in Houston TX on 2019-11-05 (details)
Part of the 2019 Tcl Conference

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Overview
Comment:Remove the (undocumented) query-planner control that prevents a "SELECT ALL" subquery in FROM clause from being implemented as a co-routine. This control was added by [a29e117d7ec], where it was called a "stop-gap".
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SHA3-256: ff2f5a31a2ac67a2fdbb25793e8013cb0e062ab90bdcba2d52a62d6d4d8b6d18
User & Date: drh 2017-09-28 17:29:24
Original Comment: Remove the (undocumented) query-planner control that prevents a "SELECT ALL" subquery in FROM clause from being implemented as a co-routine.
Context
2017-09-29
12:12
Better names for subqueries in EXPLAIN comments. check-in: 04ef40a8 user: drh tags: trunk
2017-09-28
20:06
Do not flatten subqueries that contain an ORDER BY or GROUP BY clause and can be implemented using a co-routine. check-in: 042d655d user: drh tags: prefer-coroutine-sort-subquery
17:29
Remove the (undocumented) query-planner control that prevents a "SELECT ALL" subquery in FROM clause from being implemented as a co-routine. This control was added by [a29e117d7ec], where it was called a "stop-gap". check-in: ff2f5a31 user: drh tags: trunk
16:56
Fix over-length source code lines in select.c. No logic changes. check-in: fd3267ef user: drh tags: trunk
Changes
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Changes to src/select.c.

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    }

    /* Generate code to implement the subquery
    **
    ** The subquery is implemented as a co-routine if all of these are true:
    **   (1)  The subquery is guaranteed to be the outer loop (so that it
    **        does not need to be computed more than once)
    **   (2)  The ALL keyword after SELECT is omitted.  (Applications are
    **        allowed to say "SELECT ALL" instead of just "SELECT" to disable
    **        the use of co-routines.)
    **   (3)  Co-routines are not disabled using sqlite3_test_control()
    **        with SQLITE_TESTCTRL_OPTIMIZATIONS.
    **
    ** TODO: Are there other reasons beside (1) to use a co-routine
    ** implementation?
    */
    if( i==0
     && (pTabList->nSrc==1
            || (pTabList->a[1].fg.jointype&(JT_LEFT|JT_CROSS))!=0)  /* (1) */
     && (p->selFlags & SF_All)==0                                   /* (2) */
     && OptimizationEnabled(db, SQLITE_SubqCoroutine)               /* (3) */
    ){
      /* Implement a co-routine that will return a single row of the result
      ** set on each invocation.
      */
      int addrTop = sqlite3VdbeCurrentAddr(v)+1;
      pItem->regReturn = ++pParse->nMem;







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    }

    /* Generate code to implement the subquery
    **
    ** The subquery is implemented as a co-routine if all of these are true:
    **   (1)  The subquery is guaranteed to be the outer loop (so that it
    **        does not need to be computed more than once)
    **   (2)  REMOVED (2017-09-28): The ALL keyword after SELECT is omitted.


    **   (3)  Co-routines are not disabled using sqlite3_test_control()
    **        with SQLITE_TESTCTRL_OPTIMIZATIONS.
    **
    ** TODO: Are there other reasons beside (1) to use a co-routine
    ** implementation?
    */
    if( i==0
     && (pTabList->nSrc==1
            || (pTabList->a[1].fg.jointype&(JT_LEFT|JT_CROSS))!=0)  /* (1) */
     /*** constraint removed: && (p->selFlags & SF_All)==0             (2) */
     && OptimizationEnabled(db, SQLITE_SubqCoroutine)               /* (3) */
    ){
      /* Implement a co-routine that will return a single row of the result
      ** set on each invocation.
      */
      int addrTop = sqlite3VdbeCurrentAddr(v)+1;
      pItem->regReturn = ++pParse->nMem;