Some small tuptoaster improvements from Greg Stark. Avoid unnecessary

decompression of an already-compressed external value when we have to copy
it; save a few cycles when a value is too short for compression; and
annotate various lines that are currently unreachable.

diff --git a/src/backend/access/heap/tuptoaster.c b/src/backend/access/heap/tuptoaster.c
index 1a3c01bcac9c77848403dca0767f76626346f844..2d09b773614cf64f6f5205067ac95e7ff638b31b 100644 (file)
--- a/src/backend/access/heap/tuptoaster.c
+++ b/src/backend/access/heap/tuptoaster.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/access/heap/tuptoaster.c,v 1.74 2007/04/06 04:21:41 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/access/heap/tuptoaster.c,v 1.75 2007/09/26 23:29:10 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -525,11 +525,16 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
                        /*
                         * We took care of UPDATE above, so any external value we find
                         * still in the tuple must be someone else's we cannot reuse.
-                        * Expand it to plain (and, probably, toast it again below).
+                        * Fetch it back (without decompression, unless we are forcing
+                        * PLAIN storage).  If necessary, we'll push it out as a new
+                        * external value below.
                         */
                        if (VARATT_IS_EXTERNAL(new_value))
                        {
-                               new_value = heap_tuple_untoast_attr(new_value);
+                               if (att[i]->attstorage == 'p')
+                                       new_value = heap_tuple_untoast_attr(new_value);
+                               else
+                                       new_value = heap_tuple_fetch_attr(new_value);
                                toast_values[i] = PointerGetDatum(new_value);
                                toast_free[i] = true;
                                need_change = true;
@@ -590,7 +595,7 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
                        if (toast_action[i] != ' ')
                                continue;
                        if (VARATT_IS_EXTERNAL(toast_values[i]))
-                               continue;
+                               continue;               /* can't happen, toast_action would be 'p' */
                        if (VARATT_IS_COMPRESSED(toast_values[i]))
                                continue;
                        if (att[i]->attstorage != 'x')
@@ -654,7 +659,7 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
                        if (toast_action[i] == 'p')
                                continue;
                        if (VARATT_IS_EXTERNAL(toast_values[i]))
-                               continue;
+                               continue;               /* can't happen, toast_action would be 'p' */
                        if (att[i]->attstorage != 'x' && att[i]->attstorage != 'e')
                                continue;
                        if (toast_sizes[i] > biggest_size)
@@ -703,7 +708,7 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
                        if (toast_action[i] != ' ')
                                continue;
                        if (VARATT_IS_EXTERNAL(toast_values[i]))
-                               continue;
+                               continue;               /* can't happen, toast_action would be 'p' */
                        if (VARATT_IS_COMPRESSED(toast_values[i]))
                                continue;
                        if (att[i]->attstorage != 'm')
@@ -766,7 +771,7 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
                        if (toast_action[i] == 'p')
                                continue;
                        if (VARATT_IS_EXTERNAL(toast_values[i]))
-                               continue;
+                               continue;               /* can't happen, toast_action would be 'p' */
                        if (att[i]->attstorage != 'm')
                                continue;
                        if (toast_sizes[i] > biggest_size)
@@ -1025,6 +1030,12 @@ toast_compress_datum(Datum value)
        Assert(!VARATT_IS_EXTERNAL(value));
        Assert(!VARATT_IS_COMPRESSED(value));
 
+       /*
+        * No point in wasting a palloc cycle if value is too short for compression
+        */
+       if (valsize < PGLZ_strategy_default->min_input_size)
+               return PointerGetDatum(NULL);
+
        tmp = (struct varlena *) palloc(PGLZ_MAX_OUTPUT(valsize));
        if (pglz_compress(VARDATA_ANY(value), valsize,
                                          (PGLZ_Header *) tmp, PGLZ_strategy_default) &&
@@ -1438,10 +1449,18 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset, int32 length)
        int32           chcpystrt;
        int32           chcpyend;
 
+       Assert(VARATT_IS_EXTERNAL(attr));
+
        /* Must copy to access aligned fields */
        memcpy(&toast_pointer, VARDATA_SHORT(attr),
                   sizeof(struct varatt_external));
 
+       /*
+        * It's nonsense to fetch slices of a compressed datum -- this isn't lo_*
+        * we can't return a compressed datum which is meaningful to toast later
+        */
+       Assert(!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer));
+
        attrsize = toast_pointer.va_extsize;
        totalchunks = ((attrsize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;