Paul Ramsey <pram...@cleverelephant.ca> 于2019年7月2日周二 下午10:46写道：
> This looks good to me. A little commentary around why
> pglz_maximum_compressed_size() returns a universally correct answer
> (there's no way the compressed size can ever be larger than this
> because...) would be nice for peasants like myself.
>
> If you're looking to continue down this code line in your next patch,
> the next TODO item is a little more involved: a user-land (ala
> PG_DETOAST_DATUM) iterator API for access of TOAST datums would allow
> the optimization of searching of large objects like JSONB types, and
> so on, where the thing you are looking for is not at a known location
> in the object. So, things like looking for a particular substring in a
> string, or looking for a particular key in a JSONB. "Iterate until you
> find the thing." would allow optimization of some code lines that
> currently require full decompression of the objects.
>
> P.
>
Thanks for your comment. I've updated the patch.
As for the iterator API, I've implemented a de-TOAST iterator actually[0].
And I’m looking for more of its application scenarios and perfecting it.
Any comments would be much appreciated.
Best Regards, Binguo Bao.
[0]
https://www.postgresql.org/message-id/flat/cal-ogks_onzpc9m9bxpcztmofwulcfkyecekiagxzwrl8kx...@mail.gmail.com
From 2e4e2838937ec6fa1404fe529e7ed303e391d1b2 Mon Sep 17 00:00:00 2001
From: BBG <djydew...@gmail.com>
Date: Sun, 2 Jun 2019 19:18:46 +0800
Subject: [PATCH] Optimize partial TOAST decompression
---
src/backend/access/heap/tuptoaster.c | 24 +++++++++++++++++-------
src/common/pg_lzcompress.c | 26 ++++++++++++++++++++++++++
src/include/common/pg_lzcompress.h | 1 +
3 files changed, 44 insertions(+), 7 deletions(-)
diff --git a/src/backend/access/heap/tuptoaster.c b/src/backend/access/heap/tuptoaster.c
index 55d6e91..684f1b2 100644
--- a/src/backend/access/heap/tuptoaster.c
+++ b/src/backend/access/heap/tuptoaster.c
@@ -266,6 +266,7 @@ heap_tuple_untoast_attr_slice(struct varlena *attr,
if (VARATT_IS_EXTERNAL_ONDISK(attr))
{
struct varatt_external toast_pointer;
+ int32 max_size;
VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
@@ -273,8 +274,13 @@ heap_tuple_untoast_attr_slice(struct varlena *attr,
if (!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
return toast_fetch_datum_slice(attr, sliceoffset, slicelength);
- /* fetch it back (compressed marker will get set automatically) */
- preslice = toast_fetch_datum(attr);
+ max_size = pglz_maximum_compressed_size(sliceoffset + slicelength,
+ toast_pointer.va_rawsize);
+ /*
+ * Be sure to get enough compressed slice
+ * and compressed marker will get set automatically
+ */
+ preslice = toast_fetch_datum_slice(attr, 0, max_size);
}
else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
{
@@ -2031,7 +2037,8 @@ toast_fetch_datum(struct varlena *attr)
* Reconstruct a segment of a Datum from the chunks saved
* in the toast relation
*
- * Note that this function only supports non-compressed external datums.
+ * Note that this function supports non-compressed external datums
+ * and compressed external datum slices at the start of the object.
* ----------
*/
static struct varlena *
@@ -2072,10 +2079,9 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset, int32 length)
VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
/*
- * 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
+ * It's meaningful to fetch slices at the start of a compressed datum.
*/
- Assert(!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer));
+ Assert(!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) || 0 == sliceoffset);
attrsize = toast_pointer.va_extsize;
totalchunks = ((attrsize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;
@@ -2091,7 +2097,11 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset, int32 length)
result = (struct varlena *) palloc(length + VARHDRSZ);
- SET_VARSIZE(result, length + VARHDRSZ);
+ if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer)) {
+ SET_VARSIZE_COMPRESSED(result, length + VARHDRSZ);
+ } else {
+ SET_VARSIZE(result, length + VARHDRSZ);
+ }
if (length == 0)
return result; /* Can save a lot of work at this point! */
diff --git a/src/common/pg_lzcompress.c b/src/common/pg_lzcompress.c
index 988b398..80ed17a 100644
--- a/src/common/pg_lzcompress.c
+++ b/src/common/pg_lzcompress.c
@@ -771,3 +771,29 @@ pglz_decompress(const char *source, int32 slen, char *dest,
*/
return (char *) dp - dest;
}
+
+
+
+/* ----------
+ * pglz_max_compressed_size -
+ *
+ * Calculate the maximum size of the compressed slice corresponding to the
+ * raw slice. Return the maximum size, or raw size if maximum size is larger
+ * than raw size.
+ * ----------
+ */
+int32
+pglz_maximum_compressed_size(int32 raw_slice_size, int32 raw_size)
+{
+ int32 result;
+
+ /*
+ * Use int64 to prevent overflow during calculation.
+ */
+ result = (int32)((int64)raw_slice_size * 9 + 8) / 8;
+
+ /*
+ * Note that compressed size will never be larger than raw size.
+ */
+ return result > raw_size ? raw_size : result;
+}
diff --git a/src/include/common/pg_lzcompress.h b/src/include/common/pg_lzcompress.h
index 5555764..cda3e1d 100644
--- a/src/include/common/pg_lzcompress.h
+++ b/src/include/common/pg_lzcompress.h
@@ -87,5 +87,6 @@ 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, bool check_complete);
+extern int32 pglz_maximum_compressed_size(int32 raw_slice_size, int32 raw_size);
#endif /* _PG_LZCOMPRESS_H_ */
--
2.7.4
