Add missing comment to new routine in select.c.

FossilOrigin-Name: ef2f0cf21ba61bdd29e09cf41b012a2d757683f524a252f0af7dfee7df1a1a0f

manifest

@@ -1,5 +1,5 @@
-C Experimental\soptimization\sfor\sdistinct\saggregates\s(e.g.\s"SELECT\scount(DISTINCT\s<expr)\sFROM\s...").
-D 2021-03-09T16:06:25.224
+C Add\smissing\scomment\sto\snew\sroutine\sin\sselect.c.
+D 2021-03-09T16:47:33.281
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -542,7 +542,7 @@ F src/printf.c 2b03a80d7c11bb422115dca175a18bf430e9c9dbaa0eee63b758f0c022f8f34f
 F src/random.c 80f5d666f23feb3e6665a6ce04c7197212a88384
 F src/resolve.c 688070848f0a0c41bcc545a4b4b052921d9abc29ba3102985d3d6f7595d9637c
 F src/rowset.c ba9515a922af32abe1f7d39406b9d35730ed65efab9443dc5702693b60854c92
-F src/select.c b51fbd795e5de19bd8448816eb37b098ef679f49b2572a5d570db73aaa2494a6
+F src/select.c 7741bb3b315bd9d36c01275fb7a0b319dc30b70054f46a3521acdd71e8615d07
 F src/shell.c.in af18a2e980aabe739a8188266464866fe7947b100674e07480e7ba3e37595947
 F src/sqlite.h.in 3426a080ea1f222a73e3bd91e7eacbd30570a0117c03d42c6dde606f33e5e318
 F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
@@ -1910,10 +1910,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 9c9ba36e859e330c50ed40ede4b93eeb0a5c3337240465d953a7be9115a81390
-R 05e155a5881f070d054d078349eab572
-T *branch * distinct-agg-opt
-T *sym-distinct-agg-opt *
-T -sym-trunk *
+P eb919611fd2f255e4ad1fe7db633363793169f6cf99c650eaefa48c022eb5d22
+R 9199f92495fd5d3674624db01699c96b
 U dan
-Z a27a5de08cdd8303a3cf21c478413fb4
+Z a3e310de94fb22aed527b8a42451f5c8

manifest.uuid

@@ -1 +1 @@
-eb919611fd2f255e4ad1fe7db633363793169f6cf99c650eaefa48c022eb5d22
+ef2f0cf21ba61bdd29e09cf41b012a2d757683f524a252f0af7dfee7df1a1a0f

src/select.c

@@ -740,37 +740,38 @@ static void codeOffset(
   }
 }
 
-#if 0
 /*
-** Add code that will check to make sure the N registers starting at iMem
-** form a distinct entry.  iTab is a sorting index that holds previously
-** seen combinations of the N values.  A new entry is made in iTab
-** if the current N values are new.
+** Add code that will check to make sure the array of registers starting at
+** iMem form a distinct entry. This is used by both "SELECT DISTINCT ..." and
+** distinct aggregates ("SELECT count(DISTINCT <expr>) ..."). Parameter iTab is
+** the cursor number of an ephemeral table opened by instruction iTabAddr for
+** the code generated by this routine to use. There are three strategies, based
+** on the value of parameter eTnctType:
 **
-** A jump to addrRepeat is made and the N+1 values are popped from the
-** stack if the top N elements are not distinct.
+**   WHERE_DISTINCT_UNORDERED/WHERE_DISTINCT_NOOP:
+**     The ephemeral cursor table is queries for a record identical to the
+**     record formed by the current array of registers. If one is found,
+**     jump to VM address addrRepeat. Otherwise, insert a new record into
+**     the ephemeral cursor and proceed.
+**
+**   WHERE_DISTINCT_ORDERED:
+**     In this case rows are being delivered sorted order sorted. The ephermal
+**     table is not required in this case. Instead, the current set of 
+**     registers are compared to previous row. If they match, the new row
+**     is not distinct and control jumps to VM address addrRepeat. Otherwise,
+**     the VM program proceeds with processing the new row.
+**
+**   WHERE_DISTINCT_UNIQUE:
+**     In this case it has already been determined that the rows are distinct.
+**     No special action is required.
+**
+** Parameter pEList is the list of expressions used to generated the 
+** contents of each row. It is used by this routine to determine (a) 
+** how many elements there are in the array of registers and (b) the 
+** collation sequences that should be used for the comparisons if 
+** eTnctType is WHERE_DISTINCT_ORDERED.
 */
 static void codeDistinct(
-  Parse *pParse,     /* Parsing and code generating context */
-  int iTab,          /* A sorting index used to test for distinctness */
-  int addrRepeat,    /* Jump to here if not distinct */
-  int N,             /* Number of elements */
-  int iMem           /* First element */
-){
-  Vdbe *v;
-  int r1;
-
-  v = pParse->pVdbe;
-  r1 = sqlite3GetTempReg(pParse);
-  sqlite3VdbeAddOp4Int(v, OP_Found, iTab, addrRepeat, iMem, N); VdbeCoverage(v);
-  sqlite3VdbeAddOp3(v, OP_MakeRecord, iMem, N, r1);
-  sqlite3VdbeAddOp4Int(v, OP_IdxInsert, iTab, r1, iMem, N);
-  sqlite3VdbeChangeP5(v, OPFLAG_USESEEKRESULT);
-  sqlite3ReleaseTempReg(pParse, r1);
-}
-#endif
-
-static void codeDistinct2(
   Parse *pParse,     /* Parsing and code generating context */
   int eTnctType,     /* WHERE_DISTINCT_* value */
   int iTab,          /* A sorting index used to test for distinctness */
@@ -1087,7 +1088,7 @@ static void selectInnerLoop(
   */
   if( hasDistinct ){
     assert( nResultCol==p->pEList->nExpr );
-    codeDistinct2(pParse, pDistinct->eTnctType, pDistinct->tabTnct, 
+    codeDistinct(pParse, pDistinct->eTnctType, pDistinct->tabTnct, 
         pDistinct->addrTnct, iContinue, p->pEList, regResult);
     if( pSort==0 ){
       codeOffset(v, p->iOffset, iContinue);
@@ -5748,7 +5749,7 @@ static void updateAccumulator(
       }
       testcase( nArg==0 );  /* Error condition */
       testcase( nArg>1 );   /* Also an error */
-      codeDistinct2(pParse, eDistinctType, pF->iDistinct, pF->iDistAddr, 
+      codeDistinct(pParse, eDistinctType, pF->iDistinct, pF->iDistAddr, 
           addrNext, pList, regAgg);
     }
     if( pF->pFunc->funcFlags & SQLITE_FUNC_NEEDCOLL ){