Begin an experimental refactoring to estimate the average number of bytes

in table and index rows and to use that information in query planner.
Begin by renaming WhereCost to LogEst and making that type and its
conversion routines available outside of where.c.

FossilOrigin-Name: 66c4a251d61582b47d5cbe50cbca160a9209bd06

1 files changed, 77 insertions, 0 deletions

diff --git a/src/util.c b/src/util.c
index 7ccf2a1fd..50ffd9865 100644
--- a/src/util.c
+++ b/src/util.c
@@ -1208,3 +1208,80 @@ void sqlite3FileSuffix3(const char *zBaseFilename, char *z){
   }
 }
 #endif
+
+/* 
+** Find (an approximate) sum of two LogEst values.  This computation is
+** not a simple "+" operator because LogEst is stored as a logarithmic
+** value.
+** 
+*/
+LogEst sqlite3LogEstAdd(LogEst a, LogEst b){
+  static const unsigned char x[] = {
+     10, 10,                         /* 0,1 */
+      9, 9,                          /* 2,3 */
+      8, 8,                          /* 4,5 */
+      7, 7, 7,                       /* 6,7,8 */
+      6, 6, 6,                       /* 9,10,11 */
+      5, 5, 5,                       /* 12-14 */
+      4, 4, 4, 4,                    /* 15-18 */
+      3, 3, 3, 3, 3, 3,              /* 19-24 */
+      2, 2, 2, 2, 2, 2, 2,           /* 25-31 */
+  };
+  if( a>=b ){
+    if( a>b+49 ) return a;
+    if( a>b+31 ) return a+1;
+    return a+x[a-b];
+  }else{
+    if( b>a+49 ) return b;
+    if( b>a+31 ) return b+1;
+    return b+x[b-a];
+  }
+}
+
+/*
+** Convert an integer into a LogEst.  In other words, compute a
+** good approximatation for 10*log2(x).
+*/
+LogEst sqlite3LogEst(u64 x){
+  static LogEst a[] = { 0, 2, 3, 5, 6, 7, 8, 9 };
+  LogEst y = 40;
+  if( x<8 ){
+    if( x<2 ) return 0;
+    while( x<8 ){  y -= 10; x <<= 1; }
+  }else{
+    while( x>255 ){ y += 40; x >>= 4; }
+    while( x>15 ){  y += 10; x >>= 1; }
+  }
+  return a[x&7] + y - 10;
+}
+
+#ifndef SQLITE_OMIT_VIRTUALTABLE
+/*
+** Convert a double into a LogEst
+** In other words, compute an approximation for 10*log2(x).
+*/
+LogEst sqlite3LogEstFromDouble(double x){
+  u64 a;
+  LogEst e;
+  assert( sizeof(x)==8 && sizeof(a)==8 );
+  if( x<=1 ) return 0;
+  if( x<=2000000000 ) return sqlite3LogEst((u64)x);
+  memcpy(&a, &x, 8);
+  e = (a>>52) - 1022;
+  return e*10;
+}
+#endif /* SQLITE_OMIT_VIRTUALTABLE */
+
+/*
+** Convert a LogEst into an integer.
+*/
+u64 sqlite3LogEstToInt(LogEst x){
+  u64 n;
+  if( x<10 ) return 1;
+  n = x%10;
+  x /= 10;
+  if( n>=5 ) n -= 2;
+  else if( n>=1 ) n -= 1;
+  if( x>=3 ) return (n+8)<<(x-3);
+  return (n+8)>>(3-x);
+}