Delay extraction of TIDBitmap per page offsets

Pages from the bitmap created by the TIDBitmap API can be exact or
lossy. The TIDBitmap API extracts the tuple offsets from exact pages
into an array for the convenience of the caller.

This was done in tbm_private|shared_iterate() right after advancing the
iterator. However, as long as tbm_private|shared_iterate() set a
reference to the PagetableEntry in the TBMIterateResult, the offset
extraction can be done later.

Waiting to extract the tuple offsets has a few benefits. For the shared
iterator case, it allows us to extract the offsets after dropping the
shared iterator state lock, reducing time spent holding a contended
lock.

Separating the iteration step and extracting the offsets later also
allows us to avoid extracting the offsets for prefetched blocks. Those
offsets were never used, so the overhead of extracting and storing them
was wasted.

The real motivation for this change, however, is that future commits
will make bitmap heap scan use the read stream API. This requires a
TBMIterateResult per issued block. By removing the array of tuple
offsets from the TBMIterateResult and only extracting the offsets when
they are used, we reduce the memory required for per buffer data
substantially.

Suggested-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGLHbKP3jwJ6_%2BhnGi37Pw3BD5j2amjV3oSk7j-KyCnY7Q%40mail.gmail.com

1 files changed, 19 insertions, 9 deletions

diff --git a/src/backend/access/gin/ginget.c b/src/backend/access/gin/ginget.c
index 54aecc1d1f1..f3b19d280c3 100644
--- a/src/backend/access/gin/ginget.c
+++ b/src/backend/access/gin/ginget.c
@@ -333,6 +333,7 @@ restartScanEntry:
 	entry->nlist = 0;
 	entry->matchBitmap = NULL;
 	entry->matchResult = NULL;
+	entry->matchNtuples = -1;
 	entry->reduceResult = false;
 	entry->predictNumberResult = 0;
 
@@ -828,7 +829,7 @@ entryGetItem(GinState *ginstate, GinScanEntry entry,
 			 */
 			while (entry->matchResult == NULL ||
 				   (!entry->matchResult->lossy &&
-					entry->offset >= entry->matchResult->ntuples) ||
+					entry->offset >= entry->matchNtuples) ||
 				   entry->matchResult->blockno < advancePastBlk ||
 				   (ItemPointerIsLossyPage(&advancePast) &&
 					entry->matchResult->blockno == advancePastBlk))
@@ -845,9 +846,15 @@ entryGetItem(GinState *ginstate, GinScanEntry entry,
 					break;
 				}
 
+				/* Exact pages need their tuple offsets extracted. */
+				if (!entry->matchResult->lossy)
+					entry->matchNtuples = tbm_extract_page_tuple(entry->matchResult,
+																 entry->matchOffsets,
+																 TBM_MAX_TUPLES_PER_PAGE);
+
 				/*
 				 * Reset counter to the beginning of entry->matchResult. Note:
-				 * entry->offset is still greater than matchResult->ntuples if
+				 * entry->offset is still greater than entry->matchNtuples if
 				 * matchResult is lossy.  So, on next call we will get next
 				 * result from TIDBitmap.
 				 */
@@ -874,32 +881,35 @@ entryGetItem(GinState *ginstate, GinScanEntry entry,
 			}
 
 			/*
-			 * Not a lossy page. Skip over any offsets <= advancePast, and
-			 * return that.
+			 * Not a lossy page. If tuple offsets were extracted,
+			 * entry->matchNtuples must be > -1
 			 */
+			Assert(entry->matchNtuples > -1);
+
+			/* Skip over any offsets <= advancePast, and return that. */
 			if (entry->matchResult->blockno == advancePastBlk)
 			{
-				Assert(entry->matchResult->ntuples > 0);
+				Assert(entry->matchNtuples > 0);
 
 				/*
 				 * First, do a quick check against the last offset on the
 				 * page. If that's > advancePast, so are all the other
 				 * offsets, so just go back to the top to get the next page.
 				 */
-				if (entry->matchResult->offsets[entry->matchResult->ntuples - 1] <= advancePastOff)
+				if (entry->matchOffsets[entry->matchNtuples - 1] <= advancePastOff)
 				{
-					entry->offset = entry->matchResult->ntuples;
+					entry->offset = entry->matchNtuples;
 					continue;
 				}
 
 				/* Otherwise scan to find the first item > advancePast */
-				while (entry->matchResult->offsets[entry->offset] <= advancePastOff)
+				while (entry->matchOffsets[entry->offset] <= advancePastOff)
 					entry->offset++;
 			}
 
 			ItemPointerSet(&entry->curItem,
 						   entry->matchResult->blockno,
-						   entry->matchResult->offsets[entry->offset]);
+						   entry->matchOffsets[entry->offset]);
 			entry->offset++;
 
 			/* Done unless we need to reduce the result */