diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/expr.c | 5 | ||||
| -rw-r--r-- | src/select.c | 22 | ||||
| -rw-r--r-- | src/sqliteInt.h | 4 | ||||
| -rw-r--r-- | src/wherecode.c | 6 |
4 files changed, 28 insertions, 9 deletions
diff --git a/src/expr.c b/src/expr.c index 0ccb39cc6..2257422b8 100644 --- a/src/expr.c +++ b/src/expr.c @@ -5068,8 +5068,9 @@ expr_code_doover: if( !ExprHasProperty(pExpr, EP_Collate) ){ /* A TK_COLLATE Expr node without the EP_Collate tag is a so-called ** "SOFT-COLLATE" that is added to constraints that are pushed down - ** from outer queries into sub-queries by the push-down optimization. - ** Clear subtypes as subtypes may not cross a subquery boundary. + ** from outer queries into sub-queries by the WHERE-clause push-down + ** optimization. Clear subtypes as subtypes may not cross a subquery + ** boundary. */ assert( pExpr->pLeft ); sqlite3ExprCode(pParse, pExpr->pLeft, target); diff --git a/src/select.c b/src/select.c index b13aae1f9..9c2bf3b93 100644 --- a/src/select.c +++ b/src/select.c @@ -5060,6 +5060,18 @@ static int pushDownWindowCheck(Parse *pParse, Select *pSubq, Expr *pExpr){ ** The hope is that the terms added to the inner query will make it more ** efficient. ** +** NAME AMBIGUITY +** +** This optimization is called the "WHERE-clause push-down optimization". +** +** Do not confuse this optimization with another unrelated optimization +** with a similar name: The "MySQL push-down optimization" causes WHERE +** clause terms that can be evaluated using only the index and without +** reference to the table are run first, so that if they are false, +** unnecessary table seeks are avoided. +** +** RULES +** ** Do not attempt this optimization if: ** ** (1) (** This restriction was removed on 2017-09-29. We used to @@ -5125,10 +5137,10 @@ static int pushDownWindowCheck(Parse *pParse, Select *pSubq, Expr *pExpr){ ** (9c) There is a RIGHT JOIN (or FULL JOIN) in between the ON/USING ** clause and the subquery. ** -** Without this restriction, the push-down optimization might move -** the ON/USING filter expression from the left side of a RIGHT JOIN -** over to the right side, which leads to incorrect answers. See -** also restriction (6) in sqlite3ExprIsSingleTableConstraint(). +** Without this restriction, the WHERE-clause push-down optimization +** might move the ON/USING filter expression from the left side of a +** RIGHT JOIN over to the right side, which leads to incorrect answers. +** See also restriction (6) in sqlite3ExprIsSingleTableConstraint(). ** ** (10) The inner query is not the right-hand table of a RIGHT JOIN. ** @@ -7689,7 +7701,7 @@ int sqlite3Select( #endif assert( pItem->pSelect && (pItem->pSelect->selFlags & SF_PushDown)!=0 ); }else{ - TREETRACE(0x4000,pParse,p,("Push-down not possible\n")); + TREETRACE(0x4000,pParse,p,("WHERE-lcause push-down not possible\n")); } /* Convert unused result columns of the subquery into simple NULL diff --git a/src/sqliteInt.h b/src/sqliteInt.h index d80a64609..0d33f9ee2 100644 --- a/src/sqliteInt.h +++ b/src/sqliteInt.h @@ -1906,7 +1906,7 @@ struct sqlite3 { #define SQLITE_CursorHints 0x00000400 /* Add OP_CursorHint opcodes */ #define SQLITE_Stat4 0x00000800 /* Use STAT4 data */ /* TH3 expects this value ^^^^^^^^^^ to be 0x0000800. Don't change it */ -#define SQLITE_PushDown 0x00001000 /* The push-down optimization */ +#define SQLITE_PushDown 0x00001000 /* WHERE-clause push-down opt */ #define SQLITE_SimplifyJoin 0x00002000 /* Convert LEFT JOIN to JOIN */ #define SQLITE_SkipScan 0x00004000 /* Skip-scans */ #define SQLITE_PropagateConst 0x00008000 /* The constant propagation opt */ @@ -3580,7 +3580,7 @@ struct Select { #define SF_View 0x0200000 /* SELECT statement is a view */ #define SF_NoopOrderBy 0x0400000 /* ORDER BY is ignored for this query */ #define SF_UFSrcCheck 0x0800000 /* Check pSrc as required by UPDATE...FROM */ -#define SF_PushDown 0x1000000 /* SELECT has be modified by push-down opt */ +#define SF_PushDown 0x1000000 /* Modified by WHERE-clause push-down opt */ #define SF_MultiPart 0x2000000 /* Has multiple incompatible PARTITIONs */ #define SF_CopyCte 0x4000000 /* SELECT statement is a copy of a CTE */ #define SF_OrderByReqd 0x8000000 /* The ORDER BY clause may not be omitted */ diff --git a/src/wherecode.c b/src/wherecode.c index 47ce36ce3..1f9c84f00 100644 --- a/src/wherecode.c +++ b/src/wherecode.c @@ -2474,6 +2474,12 @@ Bitmask sqlite3WhereCodeOneLoopStart( ** iLoop==3: Code all remaining expressions. ** ** An effort is made to skip unnecessary iterations of the loop. + ** + ** This optimization of causing simple query restrictions to occur before + ** more complex one is call the "push-down" optimization in MySQL. Here + ** in SQLite, the name is "MySQL push-down", since there is also another + ** totally unrelated optimization called "WHERE-clause push-down". + ** Sometimes the qualifier is omitted, resulting in an ambiguity, so beware. */ iLoop = (pIdx ? 1 : 2); do{ |