From 505dcc4fdef18710c98718685180190056b4d9ed Mon Sep 17 00:00:00 2001 From: BBG Date: Sun, 2 Jun 2019 19:18:46 +0800 Subject: [PATCH] Optimize partial TOAST decompression --- src/backend/access/heap/tuptoaster.c | 34 +++++++++++++++++++++++++++------- 1 file changed, 27 insertions(+), 7 deletions(-) diff --git a/src/backend/access/heap/tuptoaster.c b/src/backend/access/heap/tuptoaster.c index 55d6e91..89ffc2d 100644 --- a/src/backend/access/heap/tuptoaster.c +++ b/src/backend/access/heap/tuptoaster.c @@ -262,6 +262,8 @@ heap_tuple_untoast_attr_slice(struct varlena *attr, struct varlena *result; char *attrdata; int32 attrsize; + int32 needsize; + int32 max_compressed_size; if (VARATT_IS_EXTERNAL_ONDISK(attr)) { @@ -273,8 +275,22 @@ 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); + needsize = sliceoffset + slicelength; + if (needsize > (INT_MAX / 9)) + { + /* Approximate to avoid overflow */ + max_compressed_size = (needsize / 8 + 1) * 9; + } + else + { + max_compressed_size = (needsize * 9) / 8 + 1; + } + + /* + * Be sure to get enough compressed slice + * and compressed marker will get set automatically + */ + preslice = toast_fetch_datum_slice(attr, 0, max_compressed_size); } else if (VARATT_IS_EXTERNAL_INDIRECT(attr)) { @@ -2031,7 +2047,8 @@ 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 +2089,9 @@ 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 +2107,11 @@ 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! */ -- 2.7.4