Improved the comment on the block of code the provides the performance

optimization originally added by check-in [925840cfdb].  The original
check-in omitted condition 4, which was the cause of bug [30027b613b].

FossilOrigin-Name: c6506b82aa6583ccde5f673c79526d5f3920b67a

manifest

@@ -1,5 +1,5 @@
-C Possible\sfix\sfor\s30027b61.\sThere\smay\sstill\sbe\sproblems\ssurrounding\sforeign\skey\nprocessing.
-D 2017-01-04T20:13:51.856
+C Improved\sthe\scomment\son\sthe\sblock\sof\scode\sthe\sprovides\sthe\sperformance\noptimization\soriginally\sadded\sby\scheck-in\s[925840cfdb].\s\sThe\soriginal\ncheck-in\somitted\scondition\s4,\swhich\swas\sthe\scause\sof\sbug\s[30027b613b].
+D 2017-01-04T22:02:56.715
 F Makefile.in 41bd4cad981487345c4a84081074bcdb876e4b2e
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 F Makefile.msc b8ca53350ae545e3562403d5da2a69cec79308da
@@ -350,7 +350,7 @@ F src/hash.c 63d0ee752a3b92d4695b2b1f5259c4621b2cfebd
 F src/hash.h ab34c5c54a9e9de2e790b24349ba5aab3dbb4fd4
 F src/hwtime.h 747c1bbe9df21a92e9c50f3bbec1de841dc5e5da
 F src/in-operator.md 10cd8f4bcd225a32518407c2fb2484089112fd71
-F src/insert.c 1e0c2b61265518a31929a10ca28102a7911d2e69
+F src/insert.c 895995aca4c6de8118d405c6c2d5fa7fd05bdb58
 F src/legacy.c 75d3023be8f0d2b99d60f905090341a03358c58e
 F src/loadext.c 5d6642d141c07d366e43d359e94ec9de47add41d
 F src/main.c e207b81542d13b9f13d61e78ca441f9781f055b0
@@ -1542,10 +1542,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 8d670973595bd01ffc9b2ab53b965e6d6d22c573
-R 18f979ea5ceac35d70bae9aad4eb77f0
-T *branch * replace-fix
-T *sym-replace-fix *
-T -sym-trunk *
-U dan
-Z a4497840837bb26a5ad3bdf1e588e388
+P 71ccb1f4c490fdebc7008e884384e7809b849742
+R c29c3a3cb5ccbc5f6d29ccf66ed137b5
+U drh
+Z 68ffb3e06be0aa51c5e3b069bb78bd06

manifest.uuid

@@ -1 +1 @@
-71ccb1f4c490fdebc7008e884384e7809b849742
+c6506b82aa6583ccde5f673c79526d5f3920b67a

src/insert.c

@@ -1549,16 +1549,22 @@ void sqlite3GenerateConstraintChecks(
       onError = OE_Abort;
     }
 
-    if( ix==0 && pPk==pIdx && onError==OE_Replace && pPk->pNext==0 ){
-      if( 0==(db->flags&SQLITE_RecTriggers)
-       || 0==sqlite3TriggersExist(pParse, pTab, TK_DELETE, 0, 0)
-      ){
-        sqlite3VdbeResolveLabel(v, addrUniqueOk);
-        continue;
-      }
+    /* Collision detection may be omitted if all of the following are true:
+    **   (1) The conflict resolution algorithm is REPLACE
+    **   (2) The table is a WITHOUT ROWID table
+    **   (3) There are no secondary indexes on the table
+    **   (4) No delete triggers need to be fired if there is a conflict
+    */ 
+    if( (ix==0 && pIdx->pNext==0)                   /* Condition 3 */
+     && pPk==pIdx                                   /* Condition 2 */
+     && onError==OE_Replace                         /* Condition 1 */
+     && ( 0==(db->flags&SQLITE_RecTriggers) ||      /* Condition 4 */
+          0==sqlite3TriggersExist(pParse, pTab, TK_DELETE, 0, 0))
+    ){
+      sqlite3VdbeResolveLabel(v, addrUniqueOk);
+      continue;
     }
 
-    
     /* Check to see if the new index entry will be unique */
     sqlite3VdbeAddOp4Int(v, OP_NoConflict, iThisCur, addrUniqueOk,
                          regIdx, pIdx->nKeyCol); VdbeCoverage(v);