jbewing commented on code in PR #5373:
URL: https://github.com/apache/hbase/pull/5373#discussion_r1387056867
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hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/StoreFileScanner.java:
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@@ -486,52 +500,184 @@ public boolean shouldUseScanner(Scan scan, HStore store,
long oldestUnexpiredTS)
@Override
public boolean seekToPreviousRow(Cell originalKey) throws IOException {
try {
- try {
- boolean keepSeeking = false;
- Cell key = originalKey;
- do {
- Cell seekKey = PrivateCellUtil.createFirstOnRow(key);
- if (seekCount != null) seekCount.increment();
- if (!hfs.seekBefore(seekKey)) {
- this.cur = null;
- return false;
- }
- Cell curCell = hfs.getCell();
- Cell firstKeyOfPreviousRow =
PrivateCellUtil.createFirstOnRow(curCell);
-
- if (seekCount != null) seekCount.increment();
- if (!seekAtOrAfter(hfs, firstKeyOfPreviousRow)) {
- this.cur = null;
- return false;
- }
-
- setCurrentCell(hfs.getCell());
- this.stopSkippingKVsIfNextRow = true;
- boolean resultOfSkipKVs;
- try {
- resultOfSkipKVs = skipKVsNewerThanReadpoint();
- } finally {
- this.stopSkippingKVsIfNextRow = false;
- }
- if (!resultOfSkipKVs || getComparator().compareRows(cur,
firstKeyOfPreviousRow) > 0) {
- keepSeeking = true;
- key = firstKeyOfPreviousRow;
- continue;
- } else {
- keepSeeking = false;
- }
- } while (keepSeeking);
- return true;
- } finally {
- realSeekDone = true;
+ if (isFastSeekingEncoding) {
+ return seekToPreviousRowStateless(originalKey);
+ } else if (previousRow == null ||
getComparator().compareRows(previousRow, originalKey) > 0) {
+ return seekToPreviousRowWithoutHint(originalKey);
+ } else {
+ return seekToPreviousRowWithHint();
}
} catch (FileNotFoundException e) {
throw e;
} catch (IOException ioe) {
throw new IOException("Could not seekToPreviousRow " + this + " to key "
+ originalKey, ioe);
+ } finally {
+ this.realSeekDone = true;
}
}
+ /**
+ * This variant of the {@link StoreFileScanner#seekToPreviousRow(Cell)}
method requires one seek
+ * and one reseek. This method maintains state in {@link
StoreFileScanner#previousRow} which only
+ * makes sense in the context of a sequential row-by-row reverse scan.
+ * {@link StoreFileScanner#previousRow} should be reset if that is not the
case. The reasoning for
+ * why this method is faster than {@link
StoreFileScanner#seekToPreviousRowStateless(Cell)} is
+ * that seeks are slower as they need to start from the beginning of the
file, while reseeks go
+ * forward from the current position.
+ */
+ private boolean seekToPreviousRowWithHint() throws IOException {
+ do {
+ Cell firstKeyOfPreviousRow =
PrivateCellUtil.createFirstOnRow(previousRow);
+ if (!seekBeforeAndSaveKeyToPreviousRow(firstKeyOfPreviousRow)) {
+ return false;
+ }
+
+ if (!reseekAtOrAfter(firstKeyOfPreviousRow)) {
+ return false;
+ }
+
+ if (isStillAtSeekTargetAfterSkippingNewerKvs(firstKeyOfPreviousRow)) {
+ return true;
+ }
+
+ if (previousRow == null) {
+ return seekToPreviousRowWithoutHint(firstKeyOfPreviousRow);
+ }
+ } while (true);
+ }
+
+ /**
+ * This variant of the {@link StoreFileScanner#seekToPreviousRow(Cell)}
method requires two seeks
+ * and one reseek. The extra expense/seek is with the intent of speeding up
subsequent calls by
+ * using the {@link StoreFileScanner#seekToPreviousRowWithHint} which this
method seeds the state
+ * for by setting {@link StoreFileScanner#previousRow}
+ */
+ private boolean seekToPreviousRowWithoutHint(Cell originalKey) throws
IOException {
+ Cell key = originalKey;
+ do {
+ // Rewind to the cell before the beginning of this row
+ Cell keyAtBeginningOfRow = PrivateCellUtil.createFirstOnRow(key);
+ if (!seekBefore(keyAtBeginningOfRow)) {
+ return false;
+ }
+
+ // Rewind before this row and save what we find as a seek hint
+ Cell firstKeyOfPreviousRow =
PrivateCellUtil.createFirstOnRow(hfs.getCell());
+ if (!seekBeforeAndSaveKeyToPreviousRow(firstKeyOfPreviousRow)) {
+ return false;
+ }
+
+ // Seek back to the start of the previous row
+ if (!reseekAtOrAfter(firstKeyOfPreviousRow)) {
+ return false;
+ }
+
+ if (isStillAtSeekTargetAfterSkippingNewerKvs(firstKeyOfPreviousRow)) {
+ return true;
+ }
+
+ if (previousRow != null) {
Review Comment:
Yes that is a perfect understanding. The more confusing case is the case in
which we can't use a previously stashed previousRow to seek backwards. That can
only happen at the very beginning of a storefile. We choose to loop here, but I
suppose we could also `return false` directly instead of attempting another
`seekBefore`. Adding comments for this all
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