Update comments about attdisbursion. NO code change.

diff --git a/src/include/catalog/pg_attribute.h b/src/include/catalog/pg_attribute.h
index 35f76c3a61f7e869f6a47857c2e43d37676a6317..a02cc2003366dfa93eaabb687d52927909d3516d 100644 (file)
--- a/src/include/catalog/pg_attribute.h
+++ b/src/include/catalog/pg_attribute.h
@@ -7,7 +7,7 @@
  *
  * Copyright (c) 1994, Regents of the University of California
  *
- * $Id: pg_attribute.h,v 1.48 1999/07/31 19:07:25 tgl Exp $
+ * $Id: pg_attribute.h,v 1.49 1999/08/09 02:45:56 tgl Exp $
  *
  * NOTES
  *   the genbki.sh script reads this file and generates .bki
@@ -53,8 +53,15 @@ CATALOG(pg_attribute) BOOTSTRAP
 
    float4      attdisbursion;
    /*
-    * attdisbursion is the disbursion statistic of the column, or zero if
-    * the statistic has not been calculated.
+    * attdisbursion is the disbursion statistic of the column (0.0 to 1.0),
+    * or zero if the statistic has not been calculated, or -1.0
+    * if VACUUM found that the column contains no duplicate entries
+    * (in which case the disbursion should be taken as 1.0/numberOfRows
+    * for the current table size).  The -1.0 hack is useful because the
+    * number of rows may be updated more often than attdisbursion is.
+    * We assume that the column will retain its no-duplicate-entry
+    * property.  (Perhaps this should be driven off the existence of a
+    * UNIQUE index for the column, instead of being a statistical guess?)
     */
 
    int2        attlen;