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Commit48107e3

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nbtree: Rename BT_RESERVED_OFFSET_MASK.
The mask was added by commit8224de4, which introduced INCLUDE nbtreeindexes. The status bits really were reserved initially. We now use 2out of 4 of the bits for additional tuple metadata, though. Rename themask to BT_STATUS_OFFSET_MASK.Also consolidate related nbtree.h code comments about the format ofpivot tuples and posting list tuples.
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‎src/include/access/nbtree.h

Lines changed: 31 additions & 38 deletions
Original file line numberDiff line numberDiff line change
@@ -282,29 +282,27 @@ typedef struct BTMetaPageData
282282
*
283283
* We store the number of columns present inside pivot tuples by abusing
284284
* their t_tid offset field, since pivot tuples never need to store a real
285-
* offset (downlinks only need tostore ablock numberin t_tid). The
285+
* offset (pivot tuples generallystore adownlinkin t_tid, though). The
286286
* offset field only stores the number of columns/attributes when the
287287
* INDEX_ALT_TID_MASK bit is set, which doesn't count the trailing heap
288288
* TID column sometimes stored in pivot tuples -- that's represented by
289289
* the presence of BT_PIVOT_HEAP_TID_ATTR. The INDEX_ALT_TID_MASK bit in
290290
* t_info is always set on BTREE_VERSION 4 pivot tuples, since
291291
* BTreeTupleIsPivot() must work reliably on heapkeyspace versions.
292292
*
293-
* In version 3 indexes, the INDEX_ALT_TID_MASK flag might not be set in
294-
* pivot tuples. In that case, the number of key columns is implicitly
295-
* the same as the number of key columns in the index. It is not usually
296-
* set on version 2 indexes, which predate the introduction of INCLUDE
297-
* indexes. (Only explicitly truncated pivot tuples explicitly represent
298-
* the number of key columns on versions 2 and 3, whereas all pivot tuples
299-
* are formed using truncation on version 4. A version 2 index will have
300-
* it set for an internal page negative infinity item iff internal page
301-
* split occurred after upgrade to Postgres 11+.)
293+
* In version 2 or version 3 (!heapkeyspace) indexes, INDEX_ALT_TID_MASK
294+
* might not be set in pivot tuples. BTreeTupleIsPivot() won't work
295+
* reliably as a result. The number of columns stored is implicitly the
296+
* same as the number of columns in the index, just like any non-pivot
297+
* tuple. (The number of columns stored should not vary, since suffix
298+
* truncation of key columns is unsafe within any !heapkeyspace index.)
302299
*
303-
* The 12 least significant offset bits from t_tid are used to represent
304-
* the number of columns in INDEX_ALT_TID_MASK tuples, leaving 4 status
305-
* bits (BT_RESERVED_OFFSET_MASK bits), 3 of which that are reserved for
306-
* future use. BT_OFFSET_MASK should be large enough to store any number
307-
* of columns/attributes <= INDEX_MAX_KEYS.
300+
* The 12 least significant bits from t_tid's offset number are used to
301+
* represent the number of key columns within a pivot tuple. This leaves 4
302+
* status bits (BT_STATUS_OFFSET_MASK bits), which are shared by all tuples
303+
* that have the INDEX_ALT_TID_MASK bit set (set in t_info) to store basic
304+
* tuple metadata. BTreeTupleIsPivot() and BTreeTupleIsPosting() use the
305+
* BT_STATUS_OFFSET_MASK bits.
308306
*
309307
* Sometimes non-pivot tuples also use a representation that repurposes
310308
* t_tid to store metadata rather than a TID. PostgreSQL v13 introduced a
@@ -321,31 +319,24 @@ typedef struct BTMetaPageData
321319
*
322320
* Posting list tuples are recognized as such by having the
323321
* INDEX_ALT_TID_MASK status bit set in t_info and the BT_IS_POSTING status
324-
* bit set in t_tid. These flags redefine the content of the posting
325-
* tuple's t_tid to store an offset to the posting list, as well as the
326-
* total number of posting list array elements.
322+
* bit set in t_tid's offset number. These flags redefine the content of
323+
* the posting tuple's t_tid to store the location of the posting list
324+
* (instead of a block number), as well as the total number of heap TIDs
325+
* present in the tuple (instead of a real offset number).
327326
*
328-
* The 12 least significant offset bits from t_tid are used to represent
329-
* the number of posting items present in the tuple, leaving 4 status
330-
* bits (BT_RESERVED_OFFSET_MASK bits), 3 of which that are reserved for
331-
* future use. Like any non-pivot tuple, the number of columns stored is
332-
* always implicitly the total number in the index (in practice there can
333-
* never be non-key columns stored, since deduplication is not supported
334-
* with INCLUDE indexes). BT_OFFSET_MASK should be large enough to store
335-
* any number of posting list TIDs that might be present in a tuple (since
336-
* tuple size is subject to the INDEX_SIZE_MASK limit).
337-
*
338-
* Note well: The macros that deal with the number of attributes in tuples
339-
* assume that a tuple with INDEX_ALT_TID_MASK set must be a pivot tuple or
340-
* non-pivot posting tuple, and that a tuple without INDEX_ALT_TID_MASK set
341-
* must be a non-pivot tuple (or must have the same number of attributes as
342-
* the index has generally in the case of !heapkeyspace indexes).
327+
* The 12 least significant bits from t_tid's offset number are used to
328+
* represent the number of heap TIDs present in the tuple, leaving 4 status
329+
* bits (the BT_STATUS_OFFSET_MASK bits). Like any non-pivot tuple, the
330+
* number of columns stored is always implicitly the total number in the
331+
* index (in practice there can never be non-key columns stored, since
332+
* deduplication is not supported with INCLUDE indexes).
343333
*/
344334
#defineINDEX_ALT_TID_MASKINDEX_AM_RESERVED_BIT
345335

346-
/* Item pointer offset bits */
347-
#defineBT_RESERVED_OFFSET_MASK0xF000
336+
/* Item pointer offset bit masks */
348337
#defineBT_OFFSET_MASK0x0FFF
338+
#defineBT_STATUS_OFFSET_MASK0xF000
339+
/* BT_STATUS_OFFSET_MASK status bits */
349340
#defineBT_PIVOT_HEAP_TID_ATTR0x1000
350341
#defineBT_IS_POSTING0x2000
351342

@@ -378,11 +369,13 @@ BTreeTupleIsPosting(IndexTuple itup)
378369
}
379370

380371
staticinlinevoid
381-
BTreeTupleSetPosting(IndexTupleitup,intnhtids,intpostingoffset)
372+
BTreeTupleSetPosting(IndexTupleitup,uint16nhtids,intpostingoffset)
382373
{
383-
Assert(nhtids>1&& (nhtids&BT_OFFSET_MASK)==nhtids);
374+
Assert(nhtids>1);
375+
Assert((nhtids&BT_STATUS_OFFSET_MASK)==0);
384376
Assert((size_t)postingoffset==MAXALIGN(postingoffset));
385377
Assert(postingoffset<INDEX_SIZE_MASK);
378+
Assert(!BTreeTupleIsPivot(itup));
386379

387380
itup->t_info |=INDEX_ALT_TID_MASK;
388381
ItemPointerSetOffsetNumber(&itup->t_tid, (nhtids |BT_IS_POSTING));
@@ -470,7 +463,7 @@ static inline void
470463
BTreeTupleSetNAtts(IndexTupleitup,uint16nkeyatts,boolheaptid)
471464
{
472465
Assert(nkeyatts <=INDEX_MAX_KEYS);
473-
Assert((nkeyatts&BT_RESERVED_OFFSET_MASK)==0);
466+
Assert((nkeyatts&BT_STATUS_OFFSET_MASK)==0);
474467
Assert(!heaptid||nkeyatts>0);
475468
Assert(!BTreeTupleIsPivot(itup)||nkeyatts==0);
476469

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