Update the explain virtual table to make use of SQLITE_CONSTRAINT.

FossilOrigin-Name: b2d41ff7027b44ccb5dffc303c47d42f1f6fd66ce22e90450d3a666c73fe8b8e

1 files changed, 23 insertions, 11 deletions

diff --git a/ext/misc/explain.c b/ext/misc/explain.c
index dc0c9d766..7a2fc4776 100644
--- a/ext/misc/explain.c
+++ b/ext/misc/explain.c
@@ -18,6 +18,10 @@
 **     .load ./explain
 **     SELECT p2 FROM explain('SELECT * FROM sqlite_master')
 **      WHERE opcode='OpenRead';
+**
+** This module was originally written to help simplify SQLite testing,
+** by providing an easier means of verifying certain patterns in the
+** generated bytecode.
 */
 #if !defined(SQLITEINT_H)
 #include "sqlite3ext.h"
@@ -232,23 +236,31 @@ static int explainBestIndex(
   sqlite3_vtab *tab,
   sqlite3_index_info *pIdxInfo
 ){
-  int i;
+  int i;                  /* Loop counter */
+  int idx = -1;           /* Index of a usable == constraint against SQL */
+  int unusable = 0;       /* True if there are unusable constraints on SQL */
 
-  pIdxInfo->estimatedCost = (double)1000000;
   pIdxInfo->estimatedRows = 500;
   for(i=0; i<pIdxInfo->nConstraint; i++){
     struct sqlite3_index_constraint *p = &pIdxInfo->aConstraint[i];
-    if( p->usable
-     && p->iColumn==EXPLN_COLUMN_SQL
-     && p->op==SQLITE_INDEX_CONSTRAINT_EQ
-    ){
-      pIdxInfo->estimatedCost = 10.0;
-      pIdxInfo->idxNum = 1;
-      pIdxInfo->aConstraintUsage[i].argvIndex = 1;
-      pIdxInfo->aConstraintUsage[i].omit = 1;
-      break;
+    if( p->iColumn!=EXPLN_COLUMN_SQL ) continue;
+    if( !p->usable ){
+      unusable = 1;
+    }else if( p->op==SQLITE_INDEX_CONSTRAINT_EQ ){
+      idx = i;
     }
   }
+  if( idx>=0 ){
+    /* There exists a usable == constraint against the SQL column */
+    pIdxInfo->estimatedCost = 10.0;
+    pIdxInfo->idxNum = 1;
+    pIdxInfo->aConstraintUsage[idx].argvIndex = 1;
+    pIdxInfo->aConstraintUsage[idx].omit = 1;
+  }else if( unusable ){
+    /* There are unusable constraints against the SQL column.  Do not allow
+    ** this plan to continue forward. */
+    return SQLITE_CONSTRAINT;
+  }
   return SQLITE_OK;
 }