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| author | drh <drh@noemail.net> | 2016-03-15 17:52:12 +0000 |
|---|---|---|
| committer | drh <drh@noemail.net> | 2016-03-15 17:52:12 +0000 |
| commit | 0ff47e9e1b74e270648cac1ea72f9bcf4e35031d (patch) | |
| tree | 72ff372a009adf8e5cd81f51de4eddc6f0f2bc1e /src | |
| parent | 82f525406af6e0ca6887a82c8bb561fac610fa59 (diff) | |
| download | sqlite-0ff47e9e1b74e270648cac1ea72f9bcf4e35031d.tar.gz sqlite-0ff47e9e1b74e270648cac1ea72f9bcf4e35031d.zip | |
Implement FROM-clause subqueries as co-routines whenever they are guaranteed
to be the outer-most loop of the join.
FossilOrigin-Name: c7bae50bdccb5bcf3bc22e8ac5bb6725ef13db39
Diffstat (limited to 'src')
| -rw-r--r-- | src/select.c | 20 |
1 files changed, 17 insertions, 3 deletions
diff --git a/src/select.c b/src/select.c index c9bc389b2..a62581efc 100644 --- a/src/select.c +++ b/src/select.c @@ -4970,10 +4970,24 @@ int sqlite3Select( } /* 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( pTabList->nSrc==1 - && (p->selFlags & SF_All)==0 - && OptimizationEnabled(db, SQLITE_SubqCoroutine) + 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. |