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Overview
Comment: | Clean up the comments on the query flattener to more clearly and accurately express the conditions under which the flattener is able to run. |
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Downloads: | Tarball | ZIP archive |
Timelines: | family | ancestors | descendants | both | trunk |
Files: | files | file ages | folders |
SHA3-256: |
0840f9f824c16212ce3fd6c859e50117 |
User & Date: | drh 2017-09-29 16:07:56.167 |
Context
2017-10-02
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02:32 | Improvements to a comment in lempar.c. No code changes. (check-in: 8000d23017 user: drh tags: trunk) | |
2017-09-29
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16:08 | Merge the query flattener comment improvements from trunk. (check-in: f62cd4d940 user: drh tags: prefer-coroutine-sort-subquery) | |
16:07 | Clean up the comments on the query flattener to more clearly and accurately express the conditions under which the flattener is able to run. (check-in: 0840f9f824 user: drh tags: trunk) | |
12:44 | Make sure the 6th parameter to the authorizer callback for view subqueries has the correct view name. (check-in: 2a45bbc9fd user: drh tags: trunk) | |
Changes
Changes to src/select.c.
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3292 3293 3294 3295 3296 3297 3298 | ** SELECT x+y AS a FROM t1 WHERE z<100 AND a>5 ** ** The code generated for this simplification gives the same result ** but only has to scan the data once. And because indices might ** exist on the table t1, a complete scan of the data might be ** avoided. ** | | | > | | | | > | | | | > | < | | | < | | | | | | | | | | | | > | < > > | | | | | | | | | | | 3292 3293 3294 3295 3296 3297 3298 3299 3300 3301 3302 3303 3304 3305 3306 3307 3308 3309 3310 3311 3312 3313 3314 3315 3316 3317 3318 3319 3320 3321 3322 3323 3324 3325 3326 3327 3328 3329 3330 3331 3332 3333 3334 3335 3336 3337 3338 3339 3340 3341 3342 3343 3344 3345 3346 3347 3348 3349 3350 3351 3352 3353 3354 3355 3356 3357 3358 3359 3360 3361 3362 3363 3364 3365 3366 3367 3368 3369 3370 3371 3372 3373 3374 3375 3376 3377 3378 3379 3380 3381 3382 3383 3384 3385 3386 3387 3388 3389 3390 3391 3392 3393 3394 3395 3396 3397 3398 3399 3400 3401 3402 3403 3404 3405 3406 3407 3408 | ** SELECT x+y AS a FROM t1 WHERE z<100 AND a>5 ** ** The code generated for this simplification gives the same result ** but only has to scan the data once. And because indices might ** exist on the table t1, a complete scan of the data might be ** avoided. ** ** Flattening is subject to the following constraints: ** ** (1) The subquery and the outer query cannot both be aggregates. ** ** (2) If the subquery is an aggregate then ** (2a) the outer query must not be a join and ** (2b) the outer query must not use subqueries ** other than the one FROM-clause subquery that is a candidate ** for flattening. (This is due to ticket [2f7170d73bf9abf80] ** from 2015-02-09.) ** ** (3) If the subquery is the right operand of a LEFT JOIN then ** (3a) the subquery may not be a join and ** (3b) the FROM clause of the subquery may not contain a virtual ** table and ** (3c) the outer query may not be an aggregate. ** ** (4) The subquery can not be DISTINCT. ** ** (**) At one point restrictions (4) and (5) defined a subset of DISTINCT ** sub-queries that were excluded from this optimization. Restriction ** (4) has since been expanded to exclude all DISTINCT subqueries. ** ** (6) If the subquery is aggregate, the outer query may not be DISTINCT. ** ** (7) The subquery must have a FROM clause. TODO: For subqueries without ** A FROM clause, consider adding a FROM clause with the special ** table sqlite_once that consists of a single row containing a ** single NULL. ** ** (8) If the subquery uses LIMIT then the outer query may not be a join. ** ** (9) If the subquery uses LIMIT then the outer query may not be aggregate. ** ** (**) Restriction (10) was removed from the code on 2005-02-05 but we ** accidently carried the comment forward until 2014-09-15. Original ** constraint: "If the subquery is aggregate then the outer query ** may not use LIMIT." ** ** (11) The subquery and the outer query may not both have ORDER BY clauses. ** ** (**) Not implemented. Subsumed into restriction (3). Was previously ** a separate restriction deriving from ticket #350. ** ** (13) The subquery and outer query may not both use LIMIT. ** ** (14) The subquery may not use OFFSET. ** ** (15) If the outer query is part of a compound select, then the ** subquery may not use LIMIT. ** (See ticket #2339 and ticket [02a8e81d44]). ** ** (16) If the outer query is aggregate, then the subquery may not ** use ORDER BY. (Ticket #2942) This used to not matter ** until we introduced the group_concat() function. ** ** (17) If the subquery is a compound select, then ** (17a) all compound operators must be a UNION ALL, and ** (17b) no terms within the subquery compound may be aggregate ** or DISTINT, and ** (17c) every term within the subquery compound must have a FROM clause ** (17d) the outer query may not be ** (17d1) aggregate, or ** (17d2) DISTINCT, or ** (17d3) a join. ** ** The parent and sub-query may contain WHERE clauses. Subject to ** rules (11), (13) and (14), they may also contain ORDER BY, ** LIMIT and OFFSET clauses. The subquery cannot use any compound ** operator other than UNION ALL because all the other compound ** operators have an implied DISTINCT which is disallowed by ** restriction (4). ** ** Also, each component of the sub-query must return the same number ** of result columns. This is actually a requirement for any compound ** SELECT statement, but all the code here does is make sure that no ** such (illegal) sub-query is flattened. The caller will detect the ** syntax error and return a detailed message. ** ** (18) If the sub-query is a compound select, then all terms of the ** ORDER BY clause of the parent must be simple references to ** columns of the sub-query. ** ** (19) If the subquery uses LIMIT then the outer query may not ** have a WHERE clause. ** ** (20) If the sub-query is a compound select, then it must not use ** an ORDER BY clause. Ticket #3773. We could relax this constraint ** somewhat by saying that the terms of the ORDER BY clause must ** appear as unmodified result columns in the outer query. But we ** have other optimizations in mind to deal with that case. ** ** (21) If the subquery uses LIMIT then the outer query may not be ** DISTINCT. (See ticket [752e1646fc]). ** ** (22) The subquery may not be a recursive CTE. ** ** (23) If the outer query is a recursive CTE, then the sub-query may not be ** a compound query. This restriction is because transforming the ** parent to a compound query confuses the code that handles ** recursive queries in multiSelect(). ** ** (24) The subquery may not be an aggregate that uses the built-in min() or ** or max() functions. (Without this restriction, a query like: ** "SELECT x FROM (SELECT max(y), x FROM t1)" would not necessarily ** return the value X for which Y was maximal.) ** ** ** In this routine, the "p" parameter is a pointer to the outer query. ** The subquery is p->pSrc->a[iFrom]. isAgg is true if the outer query |
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3431 3432 3433 3434 3435 3436 3437 | Expr *pWhere; /* The WHERE clause */ struct SrcList_item *pSubitem; /* The subquery */ sqlite3 *db = pParse->db; /* Check to see if flattening is permitted. Return 0 if not. */ assert( p!=0 ); | | | 3434 3435 3436 3437 3438 3439 3440 3441 3442 3443 3444 3445 3446 3447 3448 | Expr *pWhere; /* The WHERE clause */ struct SrcList_item *pSubitem; /* The subquery */ sqlite3 *db = pParse->db; /* Check to see if flattening is permitted. Return 0 if not. */ assert( p!=0 ); assert( p->pPrior==0 ); if( OptimizationDisabled(db, SQLITE_QueryFlattener) ) return 0; pSrc = p->pSrc; assert( pSrc && iFrom>=0 && iFrom<pSrc->nSrc ); pSubitem = &pSrc->a[iFrom]; iParent = pSubitem->iCursor; pSub = pSubitem->pSelect; assert( pSub!=0 ); |
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3463 3464 3465 3466 3467 3468 3469 | ** and (14). */ if( pSub->pLimit && p->pLimit ) return 0; /* Restriction (13) */ if( pSub->pOffset ) return 0; /* Restriction (14) */ if( (p->selFlags & SF_Compound)!=0 && pSub->pLimit ){ return 0; /* Restriction (15) */ } if( pSubSrc->nSrc==0 ) return 0; /* Restriction (7) */ | | | 3466 3467 3468 3469 3470 3471 3472 3473 3474 3475 3476 3477 3478 3479 3480 | ** and (14). */ if( pSub->pLimit && p->pLimit ) return 0; /* Restriction (13) */ if( pSub->pOffset ) return 0; /* Restriction (14) */ if( (p->selFlags & SF_Compound)!=0 && pSub->pLimit ){ return 0; /* Restriction (15) */ } if( pSubSrc->nSrc==0 ) return 0; /* Restriction (7) */ if( pSub->selFlags & SF_Distinct ) return 0; /* Restriction (4) */ if( pSub->pLimit && (pSrc->nSrc>1 || isAgg) ){ return 0; /* Restrictions (8)(9) */ } if( (p->selFlags & SF_Distinct)!=0 && subqueryIsAgg ){ return 0; /* Restriction (6) */ } if( p->pOrderBy && pSub->pOrderBy ){ |
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3489 3490 3491 3492 3493 3494 3495 | } if( (p->selFlags & SF_Recursive) && pSub->pPrior ){ return 0; /* Restriction (23) */ } /* ** If the subquery is the right operand of a LEFT JOIN, then the | | > | | | > | | | | | | | | | 3492 3493 3494 3495 3496 3497 3498 3499 3500 3501 3502 3503 3504 3505 3506 3507 3508 3509 3510 3511 3512 3513 3514 3515 3516 3517 3518 3519 3520 3521 3522 3523 3524 3525 3526 3527 3528 3529 3530 3531 3532 3533 3534 3535 3536 3537 3538 3539 3540 3541 3542 3543 3544 3545 3546 3547 3548 3549 3550 3551 3552 3553 3554 3555 3556 3557 3558 3559 3560 3561 3562 3563 3564 3565 3566 3567 | } if( (p->selFlags & SF_Recursive) && pSub->pPrior ){ return 0; /* Restriction (23) */ } /* ** If the subquery is the right operand of a LEFT JOIN, then the ** subquery may not be a join itself (3a). Example of why this is not ** allowed: ** ** t1 LEFT OUTER JOIN (t2 JOIN t3) ** ** If we flatten the above, we would get ** ** (t1 LEFT OUTER JOIN t2) JOIN t3 ** ** which is not at all the same thing. ** ** If the subquery is the right operand of a LEFT JOIN, then the outer ** query cannot be an aggregate. (3c) This is an artifact of the way ** aggregates are processed - there is no mechanism to determine if ** the LEFT JOIN table should be all-NULL. ** ** See also tickets #306, #350, and #3300. */ if( (pSubitem->fg.jointype & JT_OUTER)!=0 ){ isLeftJoin = 1; if( pSubSrc->nSrc>1 || isAgg || IsVirtual(pSubSrc->a[0].pTab) ){ /* (3a) (3c) (3b) */ return 0; } } #ifdef SQLITE_EXTRA_IFNULLROW else if( iFrom>0 && !isAgg ){ /* Setting isLeftJoin to -1 causes OP_IfNullRow opcodes to be generated for ** every reference to any result column from subquery in a join, even ** though they are not necessary. This will stress-test the OP_IfNullRow ** opcode. */ isLeftJoin = -1; } #endif /* Restriction (17): If the sub-query is a compound SELECT, then it must ** use only the UNION ALL operator. And none of the simple select queries ** that make up the compound SELECT are allowed to be aggregate or distinct ** queries. */ if( pSub->pPrior ){ if( pSub->pOrderBy ){ return 0; /* Restriction (20) */ } if( isAgg || (p->selFlags & SF_Distinct)!=0 || pSrc->nSrc!=1 ){ return 0; /* (17d1), (17d2), or (17d3) */ } for(pSub1=pSub; pSub1; pSub1=pSub1->pPrior){ testcase( (pSub1->selFlags & (SF_Distinct|SF_Aggregate))==SF_Distinct ); testcase( (pSub1->selFlags & (SF_Distinct|SF_Aggregate))==SF_Aggregate ); assert( pSub->pSrc!=0 ); assert( pSub->pEList->nExpr==pSub1->pEList->nExpr ); if( (pSub1->selFlags & (SF_Distinct|SF_Aggregate))!=0 /* (17b) */ || (pSub1->pPrior && pSub1->op!=TK_ALL) /* (17a) */ || pSub1->pSrc->nSrc<1 /* (17c) */ ){ return 0; } testcase( pSub1->pSrc->nSrc>1 ); } /* Restriction (18). */ if( p->pOrderBy ){ int ii; for(ii=0; ii<p->pOrderBy->nExpr; ii++){ if( p->pOrderBy->a[ii].u.x.iOrderByCol==0 ) return 0; } } } |
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