Add support for partial TOAST decompression

When asked for a slice of a TOAST entry, decompress enough to return the
slice instead of decompressing the entire object.

For use cases where the slice is at, or near, the beginning of the entry,
this avoids a lot of unnecessary decompression work.

This changes the signature of pglz_decompress() by adding a boolean to
indicate if it's ok for the call to finish before consuming all of the
source or destination buffers.

Author: Paul Ramsey
Reviewed-By: Rafia Sabih, Darafei Praliaskouski, Regina Obe
Discussion: https://postgr.es/m/CACowWR07EDm7Y4m2kbhN_jnys%3DBBf9A6768RyQdKm_%3DNpkcaWg%40mail.gmail.com

3 files changed, 50 insertions, 12 deletions

diff --git a/src/backend/access/heap/tuptoaster.c b/src/backend/access/heap/tuptoaster.c
index a40cfcf1954..74e957abb72 100644
--- a/src/backend/access/heap/tuptoaster.c
+++ b/src/backend/access/heap/tuptoaster.c
@@ -75,6 +75,7 @@ static struct varlena *toast_fetch_datum(struct varlena *attr);
 static struct varlena *toast_fetch_datum_slice(struct varlena *attr,
 						int32 sliceoffset, int32 length);
 static struct varlena *toast_decompress_datum(struct varlena *attr);
+static struct varlena *toast_decompress_datum_slice(struct varlena *attr, int32 slicelength);
 static int toast_open_indexes(Relation toastrel,
 				   LOCKMODE lock,
 				   Relation **toastidxs,
@@ -301,7 +302,11 @@ heap_tuple_untoast_attr_slice(struct varlena *attr,
 	{
 		struct varlena *tmp = preslice;
 
-		preslice = toast_decompress_datum(tmp);
+		/* Decompress enough to encompass the slice and the offset */
+		if (slicelength > 0 && sliceoffset >= 0)
+			preslice = toast_decompress_datum_slice(tmp, slicelength + sliceoffset);
+		else
+			preslice = toast_decompress_datum(tmp);
 
 		if (tmp != attr)
 			pfree(tmp);
@@ -2272,9 +2277,38 @@ toast_decompress_datum(struct varlena *attr)
 	if (pglz_decompress(TOAST_COMPRESS_RAWDATA(attr),
 						VARSIZE(attr) - TOAST_COMPRESS_HDRSZ,
 						VARDATA(result),
-						TOAST_COMPRESS_RAWSIZE(attr)) < 0)
+						TOAST_COMPRESS_RAWSIZE(attr), true) < 0)
+		elog(ERROR, "compressed data is corrupted");
+
+	return result;
+}
+
+
+/* ----------
+ * toast_decompress_datum_slice -
+ *
+ * Decompress the front of a compressed version of a varlena datum.
+ * offset handling happens in heap_tuple_untoast_attr_slice.
+ * Here we just decompress a slice from the front.
+ */
+static struct varlena *
+toast_decompress_datum_slice(struct varlena *attr, int32 slicelength)
+{
+	struct varlena *result;
+	int32 rawsize;
+
+	Assert(VARATT_IS_COMPRESSED(attr));
+
+	result = (struct varlena *) palloc(slicelength + VARHDRSZ);
+
+	rawsize = pglz_decompress(TOAST_COMPRESS_RAWDATA(attr),
+						VARSIZE(attr) - TOAST_COMPRESS_HDRSZ,
+						VARDATA(result),
+						slicelength, false);
+	if (rawsize < 0)
 		elog(ERROR, "compressed data is corrupted");
 
+	SET_VARSIZE(result, rawsize + VARHDRSZ);
 	return result;
 }
 
diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index cbc7e4e7ead..9196aa3aaef 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -1425,7 +1425,7 @@ RestoreBlockImage(XLogReaderState *record, uint8 block_id, char *page)
 	{
 		/* If a backup block image is compressed, decompress it */
 		if (pglz_decompress(ptr, bkpb->bimg_len, tmp.data,
-							BLCKSZ - bkpb->hole_length) < 0)
+							BLCKSZ - bkpb->hole_length, true) < 0)
 		{
 			report_invalid_record(record, "invalid compressed image at %X/%X, block %d",
 								  (uint32) (record->ReadRecPtr >> 32),
diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index 68a6e49aeb4..f82ce92ce3d 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -1894,7 +1894,7 @@ text_starts_with(PG_FUNCTION_ARGS)
 		result = false;
 	else
 	{
-		text	   *targ1 = DatumGetTextPP(arg1);
+		text	   *targ1 = text_substring(arg1, 1, len2, false);
 		text	   *targ2 = DatumGetTextPP(arg2);
 
 		result = (memcmp(VARDATA_ANY(targ1), VARDATA_ANY(targ2),
@@ -5346,17 +5346,21 @@ text_concat_ws(PG_FUNCTION_ARGS)
 Datum
 text_left(PG_FUNCTION_ARGS)
 {
-	text	   *str = PG_GETARG_TEXT_PP(0);
-	const char *p = VARDATA_ANY(str);
-	int			len = VARSIZE_ANY_EXHDR(str);
-	int			n = PG_GETARG_INT32(1);
-	int			rlen;
+	int		n = PG_GETARG_INT32(1);
 
 	if (n < 0)
-		n = pg_mbstrlen_with_len(p, len) + n;
-	rlen = pg_mbcharcliplen(p, len, n);
+	{
+		text	   *str = PG_GETARG_TEXT_PP(0);
+		const char *p = VARDATA_ANY(str);
+		int			len = VARSIZE_ANY_EXHDR(str);
+		int			rlen;
 
-	PG_RETURN_TEXT_P(cstring_to_text_with_len(p, rlen));
+		n = pg_mbstrlen_with_len(p, len) + n;
+		rlen = pg_mbcharcliplen(p, len, n);
+		PG_RETURN_TEXT_P(cstring_to_text_with_len(p, rlen));
+	}
+	else
+		PG_RETURN_TEXT_P(text_substring(PG_GETARG_DATUM(0), 1, n, false));
 }
 
 /*