Index: src/sqlite.h.in
==================================================================
--- src/sqlite.h.in
+++ src/sqlite.h.in
@@ -8035,10 +8035,24 @@
 ** of the SQL statement that triggered the call to the [xUpdate] method of the
 ** [virtual table].
 */
 int sqlite3_vtab_on_conflict(sqlite3 *);
 
+/*
+** CAPI3REF: Determine The Collation For a Virtual Table Constraint
+**
+** This function may only be called from within a call to the [xBestIndex]
+** method of a [virtual table implementation]. 
+**
+** The first argument must be the database handle with which the virtual 
+** table is associated (the one passed to the [xConnect] or [xCreate] method 
+** to create the sqlite3_vtab object. The second argument must be an index
+** into the aConstraint[] array belonging to the sqlite3_index_info structure
+** passed to xBestIndex. This function returns a pointer to a buffer 
+** containing the name of the collation sequence for the corresponding
+** constraint.
+*/
 SQLITE_EXPERIMENTAL const char *sqlite3_vtab_collation(sqlite3*, int);
 
 /*
 ** CAPI3REF: Conflict resolution modes
 ** KEYWORDS: {conflict resolution mode}

Index: src/sqliteInt.h
==================================================================
--- src/sqliteInt.h
+++ src/sqliteInt.h
@@ -1397,11 +1397,11 @@
   int nVTrans;                  /* Allocated size of aVTrans */
   Hash aModule;                 /* populated by sqlite3_create_module() */
   VtabCtx *pVtabCtx;            /* Context for active vtab connect/create */
   VTable **aVTrans;             /* Virtual tables with open transactions */
   VTable *pDisconnect;    /* Disconnect these in next sqlite3_prepare() */
-  void *pVtabWC;                /* For sqlite3_vtab_collation() */
+  void *pBestIndexCtx;          /* For sqlite3_vtab_collation() */
 #endif
   Hash aFunc;                   /* Hash table of connection functions */
   Hash aCollSeq;                /* All collating sequences */
   BusyHandler busyHandler;      /* Busy callback */
   Db aDbStatic[2];              /* Static space for the 2 default backends */

Index: src/where.c
==================================================================
--- src/where.c
+++ src/where.c
@@ -3115,18 +3115,29 @@
 
   return rc;
 }
 
 
+/*
+** Context object used to pass information from whereLoopAddVirtual()
+** to sqlite3_vtab_collation().
+*/
 struct BestIndexCtx {
   WhereClause *pWC;
   sqlite3_index_info *pIdxInfo;
   Parse *pParse;
 };
 
+/*
+** If this function is invoked from within an xBestIndex() callback, it
+** returns a pointer to a buffer containing the name of the collation
+** sequence associated with element iCons of the sqlite3_index_info.aConstraint
+** array. Or, if iCons is out of range or there is no active xBestIndex
+** call, return NULL.
+*/
 const char *sqlite3_vtab_collation(sqlite3 *db, int iCons){
-  struct BestIndexCtx *p = (struct BestIndexCtx*)db->pVtabWC;
+  struct BestIndexCtx *p = (struct BestIndexCtx*)db->pBestIndexCtx;
   const char *zRet = 0;
   if( p && iCons>=0 && iCons<p->pIdxInfo->nConstraint ){
     int iTerm = p->pIdxInfo->aConstraint[iCons].iTermOffset;
     Expr *pX = p->pWC->a[iTerm].pExpr;
     CollSeq *pC = sqlite3BinaryCompareCollSeq(p->pParse,pX->pLeft,pX->pRight);
@@ -3200,12 +3211,12 @@
   }
 
   bic.pWC = pWC;
   bic.pIdxInfo = p;
   bic.pParse = pParse;
-  pSaved = pParse->db->pVtabWC;
-  pParse->db->pVtabWC = (void*)&bic;
+  pSaved = pParse->db->pBestIndexCtx;
+  pParse->db->pBestIndexCtx = (void*)&bic;
 
   /* First call xBestIndex() with all constraints usable. */
   WHERETRACE(0x40, ("  VirtualOne: all usable\n"));
   rc = whereLoopAddVirtualOne(pBuilder, mPrereq, ALLBITS, 0, p, mNoOmit, &bIn);
 
@@ -3279,11 +3290,11 @@
     }
   }
 
   if( p->needToFreeIdxStr ) sqlite3_free(p->idxStr);
   sqlite3DbFreeNN(pParse->db, p);
-  pParse->db->pVtabWC = pSaved;
+  pParse->db->pBestIndexCtx = pSaved;
   return rc;
 }
 #endif /* SQLITE_OMIT_VIRTUALTABLE */
 
 /*