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

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);
@@ -2272,13 +2277,42 @@ 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;
+}
+
+
 /* ----------
  * toast_open_indexes
  *

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),

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));
 }
 
 /*

src/common/pg_lzcompress.c

@@ -29,7 +29,7 @@
  *
  *			int32
  *			pglz_decompress(const char *source, int32 slen, char *dest,
- *							int32 rawsize)
+ *							int32 rawsize, bool check_complete)
  *
  *				source is the compressed input.
  *
@@ -44,6 +44,12 @@
  *
  *				rawsize is the length of the uncompressed data.
  *
+ *				check_complete is a flag to let us know if -1 should be
+ *					returned in cases where we don't reach the end of the
+ *					source or dest buffers, or not.  This should be false
+ *					if the caller is asking for only a partial result and
+ *					true otherwise.
+ *
  *				The return value is the number of bytes written in the
  *				buffer dest, or -1 if decompression fails.
  *
@@ -674,13 +680,14 @@ pglz_compress(const char *source, int32 slen, char *dest,
  * pglz_decompress -
  *
  *		Decompresses source into dest. Returns the number of bytes
- *		decompressed in the destination buffer, or -1 if decompression
- *		fails.
+ *		decompressed in the destination buffer, and *optionally*
+ *		checks that both the source and dest buffers have been
+ *		fully read and written to, respectively.
  * ----------
  */
 int32
 pglz_decompress(const char *source, int32 slen, char *dest,
-				int32 rawsize)
+						 int32 rawsize, bool check_complete)
 {
 	const unsigned char *sp;
 	const unsigned char *srcend;
@@ -701,8 +708,9 @@ pglz_decompress(const char *source, int32 slen, char *dest,
 		unsigned char ctrl = *sp++;
 		int			ctrlc;
 
-		for (ctrlc = 0; ctrlc < 8 && sp < srcend; ctrlc++)
+		for (ctrlc = 0; ctrlc < 8 && sp < srcend && dp < destend; ctrlc++)
 		{
+
 			if (ctrl & 1)
 			{
 				/*
@@ -721,25 +729,13 @@ pglz_decompress(const char *source, int32 slen, char *dest,
 				if (len == 18)
 					len += *sp++;
 
-				/*
-				 * Check for output buffer overrun, to ensure we don't clobber
-				 * memory in case of corrupt input.  Note: we must advance dp
-				 * here to ensure the error is detected below the loop.  We
-				 * don't simply put the elog inside the loop since that will
-				 * probably interfere with optimization.
-				 */
-				if (dp + len > destend)
-				{
-					dp += len;
-					break;
-				}
-
 				/*
 				 * Now we copy the bytes specified by the tag from OUTPUT to
 				 * OUTPUT. It is dangerous and platform dependent to use
 				 * memcpy() here, because the copied areas could overlap
 				 * extremely!
 				 */
+				len = Min(len, destend - dp);
 				while (len--)
 				{
 					*dp = dp[-off];
@@ -752,9 +748,6 @@ pglz_decompress(const char *source, int32 slen, char *dest,
 				 * An unset control bit means LITERAL BYTE. So we just copy
 				 * one from INPUT to OUTPUT.
 				 */
-				if (dp >= destend)	/* check for buffer overrun */
-					break;		/* do not clobber memory */
-
 				*dp++ = *sp++;
 			}
 
@@ -767,12 +760,15 @@ pglz_decompress(const char *source, int32 slen, char *dest,
 
 	/*
 	 * Check we decompressed the right amount.
+	 * If we are slicing, then we won't necessarily
+	 * be at the end of the source or dest buffers
+	 * when we hit a stop, so we don't test them.
 	 */
-	if (dp != destend || sp != srcend)
+	if (check_complete && (dp != destend || sp != srcend))
 		return -1;
 
 	/*
 	 * That's it.
 	 */
-	return rawsize;
+	return (char*)dp - dest;
 }

src/include/common/pg_lzcompress.h

@@ -86,6 +86,6 @@ extern const PGLZ_Strategy *const PGLZ_strategy_always;
 extern int32 pglz_compress(const char *source, int32 slen, char *dest,
 			  const PGLZ_Strategy *strategy);
 extern int32 pglz_decompress(const char *source, int32 slen, char *dest,
-				int32 rawsize);
+				int32 rawsize, bool check_complete);
 
 #endif							/* _PG_LZCOMPRESS_H_ */