Fix issues in [/info/1e227ad9f413227f|LIMIT/OFFSET support for virtual tables].

The first problem was reported by
[forum:/forumpost/c243b8f856|forum post c243b8f856].  That report prompted
an enhancement to the generate_series() (also included in this merge) which
in turn identified other similar issues.

FossilOrigin-Name: 5f6c079d847e3664ec5acaf1b3e989efe0d548c211ae4a18936162b36df89065

2 files changed, 32 insertions, 7 deletions

diff --git a/src/where.c b/src/where.c
index 39210ddb2..820e92249 100644
--- a/src/where.c
+++ b/src/where.c
@@ -4058,6 +4058,21 @@ static int isLimitTerm(WhereTerm *pTerm){
 }
 
 /*
+** Return true if the first nCons constraints in the pUsage array are
+** marked as in-use (have argvIndex>0). False otherwise.
+*/
+static int allConstraintsUsed(
+  struct sqlite3_index_constraint_usage *aUsage, 
+  int nCons
+){
+  int ii;
+  for(ii=0; ii<nCons; ii++){
+    if( aUsage[ii].argvIndex<=0 ) return 0;
+  }
+  return 1;
+}
+
+/*
 ** Argument pIdxInfo is already populated with all constraints that may
 ** be used by the virtual table identified by pBuilder->pNew->iTab. This
 ** function marks a subset of those constraints usable, invokes the
@@ -4197,13 +4212,20 @@ static int whereLoopAddVirtualOne(
         *pbIn = 1; assert( (mExclude & WO_IN)==0 );
       }
 
+      /* Unless pbRetryLimit is non-NULL, there should be no LIMIT/OFFSET
+      ** terms. And if there are any, they should follow all other terms. */
       assert( pbRetryLimit || !isLimitTerm(pTerm) );
-      if( isLimitTerm(pTerm) && *pbIn ){
+      assert( !isLimitTerm(pTerm) || i>=nConstraint-2 );
+      assert( !isLimitTerm(pTerm) || i==nConstraint-1 || isLimitTerm(pTerm+1) );
+
+      if( isLimitTerm(pTerm) && (*pbIn || !allConstraintsUsed(pUsage, i)) ){
         /* If there is an IN(...) term handled as an == (separate call to
         ** xFilter for each value on the RHS of the IN) and a LIMIT or
-        ** OFFSET term handled as well, the plan is unusable. Set output
-        ** variable *pbRetryLimit to true to tell the caller to retry with
-        ** LIMIT and OFFSET disabled. */
+        ** OFFSET term handled as well, the plan is unusable. Similarly,
+        ** if there is a LIMIT/OFFSET and there are other unused terms,
+        ** the plan cannot be used. In these cases set variable *pbRetryLimit
+        ** to true to tell the caller to retry with LIMIT and OFFSET 
+        ** disabled. */
         if( pIdxInfo->needToFreeIdxStr ){
           sqlite3_free(pIdxInfo->idxStr);
           pIdxInfo->idxStr = 0;
diff --git a/src/whereexpr.c b/src/whereexpr.c
index 9d1f947a0..5465dc953 100644
--- a/src/whereexpr.c
+++ b/src/whereexpr.c
@@ -1638,6 +1638,7 @@ void SQLITE_NOINLINE sqlite3WhereAddLimit(WhereClause *pWC, Select *p){
         continue;
       }
       if( pWC->a[ii].leftCursor!=iCsr ) return;
+      if( pWC->a[ii].prereqRight!=0 ) return;
     }
 
     /* Check condition (5). Return early if it is not met. */
@@ -1652,12 +1653,14 @@ void SQLITE_NOINLINE sqlite3WhereAddLimit(WhereClause *pWC, Select *p){
 
     /* All conditions are met. Add the terms to the where-clause object. */
     assert( p->pLimit->op==TK_LIMIT );
-    whereAddLimitExpr(pWC, p->iLimit, p->pLimit->pLeft,
-                      iCsr, SQLITE_INDEX_CONSTRAINT_LIMIT);
-    if( p->iOffset>0 ){
+    if( p->iOffset!=0 && (p->selFlags & SF_Compound)==0 ){
       whereAddLimitExpr(pWC, p->iOffset, p->pLimit->pRight,
                         iCsr, SQLITE_INDEX_CONSTRAINT_OFFSET);
     }
+    if( p->iOffset==0 || (p->selFlags & SF_Compound)==0 ){
+      whereAddLimitExpr(pWC, p->iLimit, p->pLimit->pLeft,
+                        iCsr, SQLITE_INDEX_CONSTRAINT_LIMIT);
+    }
   }
 }