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diff --git a/contrib/hstore/hstore_compat.c b/contrib/hstore/hstore_compat.c
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+/*
+ * contrib/hstore/hstore_compat.c
+ *
+ * Notes on old/new hstore format disambiguation.
+ *
+ * There are three formats to consider:
+ * 1) old contrib/hstore (referred to as hstore-old)
+ * 2) prerelease pgfoundry hstore
+ * 3) new contrib/hstore
+ *
+ * (2) and (3) are identical except for the HS_FLAG_NEWVERSION
+ * bit, which is set in (3) but not (2).
+ *
+ * Values that are already in format (3), or which are
+ * unambiguously in format (2), are handled by the first
+ * "return immediately" test in hstoreUpgrade().
+ *
+ * To stress a point: we ONLY get here with possibly-ambiguous
+ * values if we're doing some sort of in-place migration from an
+ * old prerelease pgfoundry hstore-new; and we explicitly don't
+ * support that without fixing up any potentially padded values
+ * first. Most of the code here is serious overkill, but the
+ * performance penalty isn't serious (especially compared to the
+ * palloc() that we have to do anyway) and the belt-and-braces
+ * validity checks provide some reassurance. (If for some reason
+ * we get a value that would have worked on the old code, but
+ * which would be botched by the conversion code, the validity
+ * checks will fail it first so we get an error rather than bad
+ * data.)
+ *
+ * Note also that empty hstores are the same in (2) and (3), so
+ * there are some special-case paths for them.
+ *
+ * We tell the difference between formats (2) and (3) as follows (but
+ * note that there are some edge cases where we can't tell; see
+ * comments in hstoreUpgrade):
+ *
+ * First, since there must be at least one entry, we look at
+ * how the bits line up. The new format looks like:
+ *
+ * 10kkkkkkkkkkkkkkkkkkkkkkkkkkkkkk  (k..k = keylen)
+ * 0nvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv  (v..v = keylen+vallen)
+ *
+ * The old format looks like one of these, depending on endianness
+ * and bitfield layout: (k..k = keylen, v..v = vallen, p..p = pos,
+ * n = isnull)
+ *
+ * kkkkkkkkkkkkkkkkvvvvvvvvvvvvvvvv
+ * nppppppppppppppppppppppppppppppp
+ *
+ * kkkkkkkkkkkkkkkkvvvvvvvvvvvvvvvv
+ * pppppppppppppppppppppppppppppppn
+ *
+ * vvvvvvvvvvvvvvvvkkkkkkkkkkkkkkkk
+ * nppppppppppppppppppppppppppppppp
+ *
+ * vvvvvvvvvvvvvvvvkkkkkkkkkkkkkkkk
+ * pppppppppppppppppppppppppppppppn   (usual i386 format)
+ *
+ * If the entry is in old format, for the first entry "pos" must be 0.
+ * We can obviously see that either keylen or vallen must be >32768
+ * for there to be any ambiguity (which is why lengths less than that
+ * are fasttracked in hstore.h) Since "pos"==0, the "v" field in the
+ * new-format interpretation can only be 0 or 1, which constrains all
+ * but three bits of the old-format's k and v fields. But in addition
+ * to all of this, the data length implied by the keylen and vallen
+ * must fit in the varlena size. So the only ambiguous edge case for
+ * hstores with only one entry occurs between a new-format entry with
+ * an excess (~32k) of padding, and an old-format entry. But we know
+ * which format to use in that case based on how we were compiled, so
+ * no actual data corruption can occur.
+ *
+ * If there is more than one entry, the requirement that keys do not
+ * decrease in length, and that positions increase contiguously, and
+ * that the end of the data not be beyond the end of the varlena
+ * itself, disambiguates in almost all other cases. There is a small
+ * set of ambiguous cases which could occur if the old-format value
+ * has a large excess of padding and just the right pattern of key
+ * sizes, but these are also handled based on how we were compiled.
+ *
+ * The otherwise undocumented function hstore_version_diag is provided
+ * for testing purposes.
+ */
+#include "postgres.h"
+
+
+#include "hstore.h"
+
+/*
+ * This is the structure used for entries in the old contrib/hstore
+ * implementation. Notice that this is the same size as the new entry
+ * (two 32-bit words per key/value pair) and that the header is the
+ * same, so the old and new versions of ARRPTR, STRPTR, CALCDATASIZE
+ * etc. are compatible.
+ *
+ * If the above statement isn't true on some bizarre platform, we're
+ * a bit hosed (see StaticAssertStmt in hstoreValidOldFormat).
+ */
+typedef struct
+{
+	uint16		keylen;
+	uint16		vallen;
+	uint32
+				valisnull:1,
+				pos:31;
+} HOldEntry;
+
+static int	hstoreValidNewFormat(HStore *hs);
+static int	hstoreValidOldFormat(HStore *hs);
+
+
+/*
+ * Validity test for a new-format hstore.
+ *	0 = not valid
+ *	1 = valid but with "slop" in the length
+ *	2 = exactly valid
+ */
+static int
+hstoreValidNewFormat(HStore *hs)
+{
+	int			count = HS_COUNT(hs);
+	HEntry	   *entries = ARRPTR(hs);
+	int			buflen = (count) ? HSE_ENDPOS(entries[2 * (count) - 1]) : 0;
+	int			vsize = CALCDATASIZE(count, buflen);
+	int			i;
+
+	if (hs->size_ & HS_FLAG_NEWVERSION)
+		return 2;
+
+	if (count == 0)
+		return 2;
+
+	if (!HSE_ISFIRST(entries[0]))
+		return 0;
+
+	if (vsize > VARSIZE(hs))
+		return 0;
+
+	/* entry position must be nondecreasing */
+
+	for (i = 1; i < 2 * count; ++i)
+	{
+		if (HSE_ISFIRST(entries[i])
+			|| (HSE_ENDPOS(entries[i]) < HSE_ENDPOS(entries[i - 1])))
+			return 0;
+	}
+
+	/* key length must be nondecreasing and keys must not be null */
+
+	for (i = 1; i < count; ++i)
+	{
+		if (HSTORE_KEYLEN(entries, i) < HSTORE_KEYLEN(entries, i - 1))
+			return 0;
+		if (HSE_ISNULL(entries[2 * i]))
+			return 0;
+	}
+
+	if (vsize != VARSIZE(hs))
+		return 1;
+
+	return 2;
+}
+
+/*
+ * Validity test for an old-format hstore.
+ *	0 = not valid
+ *	1 = valid but with "slop" in the length
+ *	2 = exactly valid
+ */
+static int
+hstoreValidOldFormat(HStore *hs)
+{
+	int			count = hs->size_;
+	HOldEntry  *entries = (HOldEntry *) ARRPTR(hs);
+	int			vsize;
+	int			lastpos = 0;
+	int			i;
+
+	if (hs->size_ & HS_FLAG_NEWVERSION)
+		return 0;
+
+	/* New format uses an HEntry for key and another for value */
+	StaticAssertStmt(sizeof(HOldEntry) == 2 * sizeof(HEntry),
+					 "old hstore format is not upward-compatible");
+
+	if (count == 0)
+		return 2;
+
+	if (count > 0xFFFFFFF)
+		return 0;
+
+	if (CALCDATASIZE(count, 0) > VARSIZE(hs))
+		return 0;
+
+	if (entries[0].pos != 0)
+		return 0;
+
+	/* key length must be nondecreasing */
+
+	for (i = 1; i < count; ++i)
+	{
+		if (entries[i].keylen < entries[i - 1].keylen)
+			return 0;
+	}
+
+	/*
+	 * entry position must be strictly increasing, except for the first entry
+	 * (which can be ""=>"" and thus zero-length); and all entries must be
+	 * properly contiguous
+	 */
+
+	for (i = 0; i < count; ++i)
+	{
+		if (entries[i].pos != lastpos)
+			return 0;
+		lastpos += (entries[i].keylen
+					+ ((entries[i].valisnull) ? 0 : entries[i].vallen));
+	}
+
+	vsize = CALCDATASIZE(count, lastpos);
+
+	if (vsize > VARSIZE(hs))
+		return 0;
+
+	if (vsize != VARSIZE(hs))
+		return 1;
+
+	return 2;
+}
+
+
+/*
+ * hstoreUpgrade: PG_DETOAST_DATUM plus support for conversion of old hstores
+ */
+HStore *
+hstoreUpgrade(Datum orig)
+{
+	HStore	   *hs = (HStore *) PG_DETOAST_DATUM(orig);
+	int			valid_new;
+	int			valid_old;
+
+	/* Return immediately if no conversion needed */
+	if (hs->size_ & HS_FLAG_NEWVERSION)
+		return hs;
+
+	/* Do we have a writable copy? If not, make one. */
+	if ((void *) hs == (void *) DatumGetPointer(orig))
+		hs = (HStore *) PG_DETOAST_DATUM_COPY(orig);
+
+	if (hs->size_ == 0 ||
+		(VARSIZE(hs) < 32768 && HSE_ISFIRST((ARRPTR(hs)[0]))))
+	{
+		HS_SETCOUNT(hs, HS_COUNT(hs));
+		HS_FIXSIZE(hs, HS_COUNT(hs));
+		return hs;
+	}
+
+	valid_new = hstoreValidNewFormat(hs);
+	valid_old = hstoreValidOldFormat(hs);
+
+	if (!valid_old || hs->size_ == 0)
+	{
+		if (valid_new)
+		{
+			/*
+			 * force the "new version" flag and the correct varlena length.
+			 */
+			HS_SETCOUNT(hs, HS_COUNT(hs));
+			HS_FIXSIZE(hs, HS_COUNT(hs));
+			return hs;
+		}
+		else
+		{
+			elog(ERROR, "invalid hstore value found");
+		}
+	}
+
+	/*
+	 * this is the tricky edge case. It is only possible in some quite extreme
+	 * cases (the hstore must have had a lot of wasted padding space at the
+	 * end). But the only way a "new" hstore value could get here is if we're
+	 * upgrading in place from a pre-release version of hstore-new (NOT
+	 * contrib/hstore), so we work off the following assumptions: 1. If you're
+	 * moving from old contrib/hstore to hstore-new, you're required to fix up
+	 * any potential conflicts first, e.g. by running ALTER TABLE ... USING
+	 * col::text::hstore; on all hstore columns before upgrading. 2. If you're
+	 * moving from old contrib/hstore to new contrib/hstore, then "new" values
+	 * are impossible here 3. If you're moving from pre-release hstore-new to
+	 * hstore-new, then "old" values are impossible here 4. If you're moving
+	 * from pre-release hstore-new to new contrib/hstore, you're not doing so
+	 * as an in-place upgrade, so there is no issue So the upshot of all this
+	 * is that we can treat all the edge cases as "new" if we're being built
+	 * as hstore-new, and "old" if we're being built as contrib/hstore.
+	 *
+	 * XXX the WARNING can probably be downgraded to DEBUG1 once this has been
+	 * beta-tested. But for now, it would be very useful to know if anyone can
+	 * actually reach this case in a non-contrived setting.
+	 */
+
+	if (valid_new)
+	{
+#ifdef HSTORE_IS_HSTORE_NEW
+		elog(WARNING, "ambiguous hstore value resolved as hstore-new");
+
+		/*
+		 * force the "new version" flag and the correct varlena length.
+		 */
+		HS_SETCOUNT(hs, HS_COUNT(hs));
+		HS_FIXSIZE(hs, HS_COUNT(hs));
+		return hs;
+#else
+		elog(WARNING, "ambiguous hstore value resolved as hstore-old");
+#endif
+	}
+
+	/*
+	 * must have an old-style value. Overwrite it in place as a new-style one.
+	 */
+	{
+		int			count = hs->size_;
+		HEntry	   *new_entries = ARRPTR(hs);
+		HOldEntry  *old_entries = (HOldEntry *) ARRPTR(hs);
+		int			i;
+
+		for (i = 0; i < count; ++i)
+		{
+			uint32		pos = old_entries[i].pos;
+			uint32		keylen = old_entries[i].keylen;
+			uint32		vallen = old_entries[i].vallen;
+			bool		isnull = old_entries[i].valisnull;
+
+			if (isnull)
+				vallen = 0;
+
+			new_entries[2 * i].entry = (pos + keylen) & HENTRY_POSMASK;
+			new_entries[2 * i + 1].entry = (((pos + keylen + vallen) & HENTRY_POSMASK)
+											| ((isnull) ? HENTRY_ISNULL : 0));
+		}
+
+		if (count)
+			new_entries[0].entry |= HENTRY_ISFIRST;
+		HS_SETCOUNT(hs, count);
+		HS_FIXSIZE(hs, count);
+	}
+
+	return hs;
+}
+
+
+PG_FUNCTION_INFO_V1(hstore_version_diag);
+Datum
+hstore_version_diag(PG_FUNCTION_ARGS)
+{
+	HStore	   *hs = (HStore *) PG_DETOAST_DATUM(PG_GETARG_DATUM(0));
+	int			valid_new = hstoreValidNewFormat(hs);
+	int			valid_old = hstoreValidOldFormat(hs);
+
+	PG_RETURN_INT32(valid_old * 10 + valid_new);
+}