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nbtree: Rename BT_RESERVED_OFFSET_MASK.

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The mask was added by commit 8224de4f, which introduced INCLUDE nbtree
indexes.  The status bits really were reserved initially.  We now use 2
out of 4 of the bits for additional tuple metadata, though.  Rename the
mask to BT_STATUS_OFFSET_MASK.

Also consolidate related nbtree.h code comments about the format of
pivot tuples and posting list tuples.
This commit is contained in:
Peter Geoghegan
2020-04-22 16:09:55 -07:00
parent de0dc1a847
commit 48107e396f
1 changed files with 31 additions and 38 deletions
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69
src/include/access/nbtree.h
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@ -282,7 +282,7 @@ typedef struct BTMetaPageData
* *
* We store the number of columns present inside pivot tuples by abusing * We store the number of columns present inside pivot tuples by abusing
* their t_tid offset field, since pivot tuples never need to store a real * their t_tid offset field, since pivot tuples never need to store a real
* offset (downlinks only need to store a block number in t_tid). The * offset (pivot tuples generally store a downlink in t_tid, though). The
* offset field only stores the number of columns/attributes when the * offset field only stores the number of columns/attributes when the
* INDEX_ALT_TID_MASK bit is set, which doesn't count the trailing heap * INDEX_ALT_TID_MASK bit is set, which doesn't count the trailing heap
* TID column sometimes stored in pivot tuples -- that's represented by * TID column sometimes stored in pivot tuples -- that's represented by
@ -290,21 +290,19 @@ typedef struct BTMetaPageData
* t_info is always set on BTREE_VERSION 4 pivot tuples, since * t_info is always set on BTREE_VERSION 4 pivot tuples, since
* BTreeTupleIsPivot() must work reliably on heapkeyspace versions. * BTreeTupleIsPivot() must work reliably on heapkeyspace versions.
* *
* In version 3 indexes, the INDEX_ALT_TID_MASK flag might not be set in * In version 2 or version 3 (!heapkeyspace) indexes, INDEX_ALT_TID_MASK
* pivot tuples. In that case, the number of key columns is implicitly * might not be set in pivot tuples. BTreeTupleIsPivot() won't work
* the same as the number of key columns in the index. It is not usually * reliably as a result. The number of columns stored is implicitly the
* set on version 2 indexes, which predate the introduction of INCLUDE * same as the number of columns in the index, just like any non-pivot
* indexes. (Only explicitly truncated pivot tuples explicitly represent * tuple. (The number of columns stored should not vary, since suffix
* the number of key columns on versions 2 and 3, whereas all pivot tuples * truncation of key columns is unsafe within any !heapkeyspace index.)
* are formed using truncation on version 4. A version 2 index will have
* it set for an internal page negative infinity item iff internal page
* split occurred after upgrade to Postgres 11+.)
* *
* The 12 least significant offset bits from t_tid are used to represent * The 12 least significant bits from t_tid's offset number are used to
* the number of columns in INDEX_ALT_TID_MASK tuples, leaving 4 status * represent the number of key columns within a pivot tuple. This leaves 4
* bits (BT_RESERVED_OFFSET_MASK bits), 3 of which that are reserved for * status bits (BT_STATUS_OFFSET_MASK bits), which are shared by all tuples
* future use. BT_OFFSET_MASK should be large enough to store any number * that have the INDEX_ALT_TID_MASK bit set (set in t_info) to store basic
* of columns/attributes <= INDEX_MAX_KEYS. * tuple metadata. BTreeTupleIsPivot() and BTreeTupleIsPosting() use the
* BT_STATUS_OFFSET_MASK bits.
* *
* Sometimes non-pivot tuples also use a representation that repurposes * Sometimes non-pivot tuples also use a representation that repurposes
* t_tid to store metadata rather than a TID. PostgreSQL v13 introduced a * t_tid to store metadata rather than a TID. PostgreSQL v13 introduced a
@ -321,31 +319,24 @@ typedef struct BTMetaPageData
* *
* Posting list tuples are recognized as such by having the * Posting list tuples are recognized as such by having the
* INDEX_ALT_TID_MASK status bit set in t_info and the BT_IS_POSTING status * INDEX_ALT_TID_MASK status bit set in t_info and the BT_IS_POSTING status
* bit set in t_tid. These flags redefine the content of the posting * bit set in t_tid's offset number. These flags redefine the content of
* tuple's t_tid to store an offset to the posting list, as well as the * the posting tuple's t_tid to store the location of the posting list
* total number of posting list array elements. * (instead of a block number), as well as the total number of heap TIDs
* present in the tuple (instead of a real offset number).
* *
* The 12 least significant offset bits from t_tid are used to represent * The 12 least significant bits from t_tid's offset number are used to
* the number of posting items present in the tuple, leaving 4 status * represent the number of heap TIDs present in the tuple, leaving 4 status
* bits (BT_RESERVED_OFFSET_MASK bits), 3 of which that are reserved for * bits (the BT_STATUS_OFFSET_MASK bits). Like any non-pivot tuple, the
* future use. Like any non-pivot tuple, the number of columns stored is * number of columns stored is always implicitly the total number in the
* always implicitly the total number in the index (in practice there can * index (in practice there can never be non-key columns stored, since
* never be non-key columns stored, since deduplication is not supported * deduplication is not supported with INCLUDE indexes).
* with INCLUDE indexes). BT_OFFSET_MASK should be large enough to store
* any number of posting list TIDs that might be present in a tuple (since
* tuple size is subject to the INDEX_SIZE_MASK limit).
*
* Note well: The macros that deal with the number of attributes in tuples
* assume that a tuple with INDEX_ALT_TID_MASK set must be a pivot tuple or
* non-pivot posting tuple, and that a tuple without INDEX_ALT_TID_MASK set
* must be a non-pivot tuple (or must have the same number of attributes as
* the index has generally in the case of !heapkeyspace indexes).
*/ */
#define INDEX_ALT_TID_MASK INDEX_AM_RESERVED_BIT #define INDEX_ALT_TID_MASK INDEX_AM_RESERVED_BIT
/* Item pointer offset bits */ /* Item pointer offset bit masks */
#define BT_RESERVED_OFFSET_MASK 0xF000
#define BT_OFFSET_MASK 0x0FFF #define BT_OFFSET_MASK 0x0FFF
#define BT_STATUS_OFFSET_MASK 0xF000
/* BT_STATUS_OFFSET_MASK status bits */
#define BT_PIVOT_HEAP_TID_ATTR 0x1000 #define BT_PIVOT_HEAP_TID_ATTR 0x1000
#define BT_IS_POSTING 0x2000 #define BT_IS_POSTING 0x2000
@ -378,11 +369,13 @@ BTreeTupleIsPosting(IndexTuple itup)
} }
static inline void static inline void
BTreeTupleSetPosting(IndexTuple itup, int nhtids, int postingoffset) BTreeTupleSetPosting(IndexTuple itup, uint16 nhtids, int postingoffset)
{ {
Assert(nhtids > 1 && (nhtids & BT_OFFSET_MASK) == nhtids); Assert(nhtids > 1);
Assert((nhtids & BT_STATUS_OFFSET_MASK) == 0);
Assert((size_t) postingoffset == MAXALIGN(postingoffset)); Assert((size_t) postingoffset == MAXALIGN(postingoffset));
Assert(postingoffset < INDEX_SIZE_MASK); Assert(postingoffset < INDEX_SIZE_MASK);
Assert(!BTreeTupleIsPivot(itup));
itup->t_info |= INDEX_ALT_TID_MASK; itup->t_info |= INDEX_ALT_TID_MASK;
ItemPointerSetOffsetNumber(&itup->t_tid, (nhtids | BT_IS_POSTING)); ItemPointerSetOffsetNumber(&itup->t_tid, (nhtids | BT_IS_POSTING));
@ -470,7 +463,7 @@ static inline void
BTreeTupleSetNAtts(IndexTuple itup, uint16 nkeyatts, bool heaptid) BTreeTupleSetNAtts(IndexTuple itup, uint16 nkeyatts, bool heaptid)
{ {
Assert(nkeyatts <= INDEX_MAX_KEYS); Assert(nkeyatts <= INDEX_MAX_KEYS);
Assert((nkeyatts & BT_RESERVED_OFFSET_MASK) == 0); Assert((nkeyatts & BT_STATUS_OFFSET_MASK) == 0);
Assert(!heaptid || nkeyatts > 0); Assert(!heaptid || nkeyatts > 0);
Assert(!BTreeTupleIsPivot(itup) || nkeyatts == 0); Assert(!BTreeTupleIsPivot(itup) || nkeyatts == 0);