Make contrib/btree_gist's GiST penalty function a bit saner.

The previous coding supposed that the first differing bytes in two varlena
datums must have the same sign difference as their overall comparison
result.  This is obviously bogus for text strings in non-C locales, and
probably wrong for numeric, and even for bytea I think it was wrong on
machines where char is signed.  When the assumption failed, the function
could deliver a zero or negative penalty in situations where such a result
is quite ridiculous, leading the core GiST code to make very bad page-split
decisions.

To fix, take the absolute values of the byte-level differences.  Also,
switch the code to using unsigned char not just char, so that the behavior
will be consistent whether char is signed or not.

Per investigation of a trouble report from Tomas Vondra.  Back-patch to all
supported branches.

diff --git a/contrib/btree_gist/btree_utils_var.c b/contrib/btree_gist/btree_utils_var.c
index 7e6a3c3508afbc4ef8b4651fd058af08e31fedc4..30886cb78809a11976fed943e2651482d4048d0c 100644 (file)
--- a/contrib/btree_gist/btree_utils_var.c
+++ b/contrib/btree_gist/btree_utils_var.c
@@ -92,14 +92,12 @@ gbt_var_leaf2node(GBT_VARKEY *leaf, const gbtree_vinfo *tinfo)
 static int32
 gbt_var_node_cp_len(const GBT_VARKEY *node, const gbtree_vinfo *tinfo)
 {
-
        GBT_VARKEY_R r = gbt_var_key_readable(node);
        int32           i = 0;
        int32           l = 0;
        int32           t1len = VARSIZE(r.lower) - VARHDRSZ;
        int32           t2len = VARSIZE(r.upper) - VARHDRSZ;
        int32           ml = Min(t1len, t2len);
-
        char       *p1 = VARDATA(r.lower);
        char       *p2 = VARDATA(r.upper);
 
@@ -110,7 +108,6 @@ gbt_var_node_cp_len(const GBT_VARKEY *node, const gbtree_vinfo *tinfo)
        {
                if (tinfo->eml > 1 && l == 0)
                {
-
                        if ((l = pg_mblen(p1)) != pg_mblen(p2))
                        {
                                return i;
@@ -378,13 +375,14 @@ gbt_var_penalty(float *res, const GISTENTRY *o, const GISTENTRY *n, const gbtree
        GBT_VARKEY *newe = (GBT_VARKEY *) DatumGetPointer(n->key);
        GBT_VARKEY_R ok,
                                nk;
-       GBT_VARKEY *tmp = NULL;
 
        *res = 0.0;
 
        nk = gbt_var_key_readable(newe);
        if (nk.lower == nk.upper)       /* leaf */
        {
+               GBT_VARKEY *tmp;
+
                tmp = gbt_var_leaf2node(newe, tinfo);
                if (tmp != newe)
                        nk = gbt_var_key_readable(tmp);
@@ -401,7 +399,7 @@ gbt_var_penalty(float *res, const GISTENTRY *o, const GISTENTRY *n, const gbtree
                           ))
        {
                Datum           d = PointerGetDatum(0);
-               double          dres = 0.0;
+               double          dres;
                int32           ol,
                                        ul;
 
@@ -412,20 +410,18 @@ gbt_var_penalty(float *res, const GISTENTRY *o, const GISTENTRY *n, const gbtree
 
                if (ul < ol)
                {
-                       dres = (ol - ul);       /* lost of common prefix len */
+                       dres = (ol - ul);       /* reduction of common prefix len */
                }
                else
                {
                        GBT_VARKEY_R uk = gbt_var_key_readable((GBT_VARKEY *) DatumGetPointer(d));
+                       unsigned char tmp[4];
 
-                       char            tmp[4];
-
-                       tmp[0] = ((VARSIZE(ok.lower) - VARHDRSZ) == ul) ? (CHAR_MIN) : (VARDATA(ok.lower)[ul]);
-                       tmp[1] = ((VARSIZE(uk.lower) - VARHDRSZ) == ul) ? (CHAR_MIN) : (VARDATA(uk.lower)[ul]);
-                       tmp[2] = ((VARSIZE(ok.upper) - VARHDRSZ) == ul) ? (CHAR_MIN) : (VARDATA(ok.upper)[ul]);
-                       tmp[3] = ((VARSIZE(uk.upper) - VARHDRSZ) == ul) ? (CHAR_MIN) : (VARDATA(uk.upper)[ul]);
-                       dres = (tmp[0] - tmp[1]) +
-                               (tmp[3] - tmp[2]);
+                       tmp[0] = (unsigned char) (((VARSIZE(ok.lower) - VARHDRSZ) <= ul) ? 0 : (VARDATA(ok.lower)[ul]));
+                       tmp[1] = (unsigned char) (((VARSIZE(uk.lower) - VARHDRSZ) <= ul) ? 0 : (VARDATA(uk.lower)[ul]));
+                       tmp[2] = (unsigned char) (((VARSIZE(ok.upper) - VARHDRSZ) <= ul) ? 0 : (VARDATA(ok.upper)[ul]));
+                       tmp[3] = (unsigned char) (((VARSIZE(uk.upper) - VARHDRSZ) <= ul) ? 0 : (VARDATA(uk.upper)[ul]));
+                       dres = Abs(tmp[0] - tmp[1]) + Abs(tmp[3] - tmp[2]);
                        dres /= 256.0;
                }