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Overview
| Comment: | Remove an optimization that can no longer occur, being superceded by the subquery-reuse optimization. Put an assert in place of the optimization to detect if the need for this optimization ever returns. |
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| Downloads: | Tarball | ZIP archive | SQL archive |
| Timelines: | family | ancestors | descendants | both | reuse-subqueries |
| Files: | files | file ages | folders |
| SHA3-256: |
4fcdc7a2939457e51aab37cab168677c |
| User & Date: | drh 2018-12-31 20:13:12 |
Context
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2018-12-31
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| 20:39 | Additional steps to help ensure that scalar subqueries are only evaluated once even if they are used in multiple places within the query. This fixes a performance regression reported on the mailing list and caused by check-in [531eca6104e41e43] which was a fix for ticket [787fa716be3a7f650c]. Think of this check-in as an improved fix for that ticket. check-in: e1303193 user: drh tags: trunk | |
| 20:13 | Remove an optimization that can no longer occur, being superceded by the subquery-reuse optimization. Put an assert in place of the optimization to detect if the need for this optimization ever returns. Closed-Leaf check-in: 4fcdc7a2 user: drh tags: reuse-subqueries | |
| 18:30 | Merge enhancements and bug fixes from trunk. check-in: 9fb646f2 user: drh tags: reuse-subqueries | |
Changes
Changes to src/select.c.
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#if !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW)
/* Generate code for all sub-queries in the FROM clause
*/
pSub = pItem->pSelect;
if( pSub==0 ) continue;
/* Sometimes the code for a subquery will be generated more than
** once, if the subquery is part of the WHERE clause in a LEFT JOIN,
** for example. In that case, do not regenerate the code to manifest
** a view or the co-routine to implement a view. The first instance
** is sufficient, though the subroutine to manifest the view does need
** to be invoked again. */
if( pItem->addrFillSub ){
if( pItem->fg.viaCoroutine==0 ){
/* The subroutine that manifests the view might be a one-time routine,
** or it might need to be rerun on each iteration because it
** encodes a correlated subquery. */
testcase( sqlite3VdbeGetOp(v, pItem->addrFillSub)->opcode==OP_Once );
sqlite3VdbeAddOp2(v, OP_Gosub, pItem->regReturn, pItem->addrFillSub);
}
continue;
}
/* Increment Parse.nHeight by the height of the largest expression
** tree referred to by this, the parent select. The child select
** may contain expression trees of at most
** (SQLITE_MAX_EXPR_DEPTH-Parse.nHeight) height. This is a bit
** more conservative than necessary, but much easier than enforcing
** an exact limit.
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#if !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW)
/* Generate code for all sub-queries in the FROM clause
*/
pSub = pItem->pSelect;
if( pSub==0 ) continue;
/* The code for a subquery should only be generated once, though it is
** technically harmless for it to be generated multiple times. The
** following assert() will detect if something changes to cause
** the same subquery to be coded multiple times, as a signal to the
** developers to try to optimize the situation. */
assert( pItem->addrFillSub==0 );
/* Increment Parse.nHeight by the height of the largest expression
** tree referred to by this, the parent select. The child select
** may contain expression trees of at most
** (SQLITE_MAX_EXPR_DEPTH-Parse.nHeight) height. This is a bit
** more conservative than necessary, but much easier than enforcing
** an exact limit.
|