Becker Ewing created HBASE-28043: ------------------------------------ Summary: Reduce seeks from beginning of block in StoreFileScanner.seekToPreviousRow Key: HBASE-28043 URL: https://issues.apache.org/jira/browse/HBASE-28043 Project: HBase Issue Type: Improvement Reporter: Becker Ewing Attachments: Current_SeekToPreviousRowBehavior.png, Proposed_SeekToPreviousRowBehavior.png
Currently, for non-RIV1 DBE encodings, each call to [StoreFileScanner.seekToPreviousRow|https://github.com/apache/hbase/blob/89ca7f4ade84c84a246281c71898543b6161c099/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/StoreFileScanner.java#L493-L506] (a common operation in reverse scans) results in two seeks: # Seek from the beginning of the block to before the given row to find the prior row # Seek from the beginning of the block to the first cell of the prior row So if there are "N" rows in a block, a reverse scan through each row results in seeking past 2(N-1)! rows. This is a particularly expensive operation for tall tables that have many rows in a block. By introducing a state variable "previousRow" to StoreFileScanner, I believe that we could modify the seeking algorithm to be: # Seek from the beginning of the block to before the given row to find the prior row # Seek from the beginning of the block to before the row that is before the row that was just seeked to (i.e. 2 rows back). _Save_ this as a hint for where the prior row is in "previousRow" # Reseek from "previousRow" (2 rows back from start) to 1 row back from start (to the actual previousRow) Then the rest of the calls where a "previousRow" is present, you just need to seek to the beginning of the block once instead of twice, i.e. # seek from the beginning of the block to right before the beginning of your "previousRow" marker. Save this as the new "previousRow" marker # Reseek to the next row (i.e. your previous "previousRow" marker) See the attached diagrams for the current and proposed behavior. -- This message was sent by Atlassian Jira (v8.20.10#820010)