[Issue #1263] revise docs and fix minor problems for main index

pixels-common/src/main/java/io/pixelsdb/pixels/common/index/LocalIndexService.java

@@ -495,6 +495,10 @@ public boolean purgeIndexEntries(long tableId, long indexId, List<IndexProto.Ind
                         rowLocationFirst.getRgRowOffset(),
                         rowLocationLast.getRgRowOffset()
                 );
+                if (mainIndex.hasCache())
+                {
+                    mainIndex.flushCache(rowLocationFirst.getFileId());
+                }
                 mainIndex.deleteRowIdRange(rowIdRange);
             }
             return true;

pixels-common/src/main/java/io/pixelsdb/pixels/common/index/MainIndex.java

@@ -38,7 +38,7 @@
 public interface MainIndex extends Closeable
 {
     /**
-     * If we want to add more single point index schemes here, modify this enum.
+     * If we want to add more main index schemes, modify this enum.
      */
     enum Scheme
     {
@@ -83,7 +83,7 @@ public boolean equals(Scheme other)
     }
 
     /**
-     * @return the tableId of this mainIndex
+     * @return the table id of this main index
      */
     long getTableId();
 
@@ -97,7 +97,7 @@ public boolean equals(Scheme other)
     boolean hasCache();
 
     /**
-     * Allocate rowId batch for single point index.
+     * Allocate row id batch for single point index.
      * <br/><b>For better performance, use consistent numRowIds when calling this method.</b>
      * @param tableId the table id of single point index
      * @param numRowIds the number of row ids to allocate
@@ -136,7 +136,8 @@ public boolean equals(Scheme other)
 
     /**
      * Delete a range of row ids from the main index. This method only has effect on the persistent storage
-     * of the main index.
+     * of the main index. {@link #flushCache(long fileId)} should be called before this method
+     * delete the row ids from both cache and persistent storage.
      * {@link #getLocation(long)} of a row id within a deleted range returns null.
      * @param rowIdRange the row id range to be deleted
      * @return true on success
@@ -154,7 +155,7 @@ public boolean equals(Scheme other)
     boolean flushCache(long fileId) throws MainIndexException;
 
     /**
-     * Flush the main index cache if exists and close the main index instance.
+     * Flush the main index cache (if exists) and close the main index instance.
      * This method is to be used by the main index factory to close the
      * managed main index instances when the process is shutting down.
      * <p/>

pixels-common/src/main/java/io/pixelsdb/pixels/common/index/MainIndexBuffer.java

@@ -125,8 +125,7 @@ protected IndexProto.RowLocation lookup(long fileId, long rowId) throws MainInde
      */
     public IndexProto.RowLocation lookup(long rowId) throws MainIndexException
     {
-        IndexProto.RowLocation location = null;
-        location = this.indexCache.lookup(rowId);
+        IndexProto.RowLocation location = this.indexCache.lookup(rowId);
         if (location == null)
         {
             for (Map.Entry<Long, Map<Long, IndexProto.RowLocation>> entry : this.indexBuffer.entrySet())

pixels-common/src/main/java/io/pixelsdb/pixels/common/index/MainIndexCache.java

@@ -52,6 +52,11 @@ public class MainIndexCache implements Closeable
      */
     private final List<Map<Long, IndexProto.RowLocation>> entryCacheBuckets;
 
+    /**
+     * Caches the ranges of row ids and the corresponding locations. The cache elements are of the {@link RowIdRange}
+     * type. However, we use {@link Object} to support looking up single row ids in this range cache.
+     * Ranges in this cache may overlap but not duplicate.
+     */
     private final TreeSet<Object> rangeCache;
 
     public MainIndexCache()
@@ -124,13 +129,17 @@ public void evictAllEntries()
         }
     }
 
+    /**
+     * Admitting a row id range that is overlapping but not duplicate with existing ranges in this cache is acceptable.
+     * @param range the range to admit into the cache.
+     */
     public void admitRange(RowIdRange range)
     {
         this.rangeCache.add(range);
     }
 
     /**
-     * @param range the range to delete
+     * @param range the range to delete from the cache
      * @return true if the range is found and deleted
      */
     public boolean evictRange(RowIdRange range)

pixels-core/src/main/java/io/pixelsdb/pixels/core/vector/VectorColumnVector.java

@@ -1,3 +1,22 @@
+/*
+ * Copyright 2023 PixelsDB.
+ *
+ * This file is part of Pixels.
+ *
+ * Pixels is free software: you can redistribute it and/or modify
+ * it under the terms of the Affero GNU General Public License as
+ * published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version.
+ *
+ * Pixels is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * Affero GNU General Public License for more details.
+ *
+ * You should have received a copy of the Affero GNU General Public
+ * License along with Pixels.  If not, see
+ * <https://www.gnu.org/licenses/>.
+ */
 package io.pixelsdb.pixels.core.vector;
 
 import com.google.flatbuffers.FlatBufferBuilder;

pixels-daemon/src/main/java/io/pixelsdb/pixels/daemon/index/IndexServiceImpl.java

@@ -612,6 +612,10 @@ public void purgeIndexEntries(IndexProto.PurgeIndexEntriesRequest request,
                             rowLocationFirst.getRgId(),
                             rowLocationFirst.getRgRowOffset(),
                             rowLocationLast.getRgRowOffset());
+                    if (mainIndex.hasCache())
+                    {
+                        mainIndex.flushCache(rowLocationFirst.getFileId());
+                    }
                     mainIndex.deleteRowIdRange(rowIdRange);
                 }
                 else

pixels-daemon/src/main/java/io/pixelsdb/pixels/daemon/transaction/TransServiceImpl.java

@@ -451,7 +451,7 @@ private static void pushWatermarks(boolean readOnly)
                     logger.error("new running transactions obtained backward timestamps, this is illegal");
                     return;
                 }
-                // ush watermark to the new min running transaction timestamp
+                // push watermark to the new min running transaction timestamp
                 timestamp = minRunningTransTimestamp;
             }
             if (timestamp < 0)

pixels-index/pixels-index-main-sqlite/src/main/java/io/pixelsdb/pixels/index/main/sqlite/SqliteMainIndex.java

@@ -248,10 +248,11 @@ private IndexProto.RowLocation getRowLocationFromSqlite(long rowId) throws MainI
         try
         {
             RowIdRange rowIdRange = getRowIdRangeFromSqlite(rowId);
-            // Issue #1150: add the range to cache to accelerate main index lookups
-            this.indexCache.admitRange(rowIdRange);
             if (rowIdRange != null)
             {
+                // Issue #1150: add the range to cache to accelerate main index lookups
+                this.indexCache.admitRange(rowIdRange);
+
                 long rowIdStart = rowIdRange.getRowIdStart();
                 long fileId = rowIdRange.getFileId();
                 int rgId = rowIdRange.getRgId();