[CASSANDRA-6447] SELECT someColumns FROM table results in AssertionError in AbstractQueryPager.discardFirst

• Type: Bug
• Status: Resolved
• Priority: Normal
• Resolution: Fixed
• Fix Version/s: 2.0.4

Environment

Cluster: single node server (ubuntu)
Cassandra version: 2.0.3 (server/client)
Client: Datastax cassandra-driver-core 2.0.0-rc1

• Severity: Normal
• Since Version: 2.0.3
• Assignee: Sylvain Lebresne
• Reporter: Julien Aymé

Authors

Sylvain Lebresne

Reviewers

Aleksey Yeschenko

• Created: 04/Dec/13 11:50
• Updated: 16/Apr/19 09:31
• Resolved: 20/Dec/13 07:50

Description

I have a query which must read all the rows from the table:
Query: "SELECT key, col1, col2, col3 FROM mytable"

Here is the corresponding code (this is using datastax driver):

ResultSet result = session.execute("SELECT key, col1, col2, col3 FROM mytable");
for (Row row : result) {
     // do some work with row
}

Messages sent from the client to Cassandra:

• 1st: QUERY SELECT key, col1, col2, col3 FROM mytable([cl=ONE, vals=[], skip=false, psize=5000, state=null, serialCl=ONE])

• 2nd: QUERY SELECT key, col1, col2, col3 FROM mytable([cl=ONE, vals=[], skip=false, psize=5000, state=java.nio.HeapByteBuffer[pos=24 lim=80 cap=410474], serialCl=ONE])

On the first message, everything is fine, and the server returns 5000 rows.
On the second message, paging is in progress, and the server fails in AbstractQueryPager.discardFirst: AssertionError (stack trace attached).

Here is some more info (step by step debugging on reception of 2nd message):

AbstractQueryPager.fetchPage(int):
* pageSize=5000, currentPageSize=5001, rows size=5002, liveCount=5001
* containsPreviousLast(rows.get(0)) returns true

-> AbstractQueryPager.discardFirst(List<Row>):
* rows size=5002
* first=TreeMapBackedSortedColumns[with TreeMap size=1]

-> AbstractQueryPager.discardHead(ColumnFamily, ...):
* counter = ColumnCounter$GroupByPrefix
* iter.hasNext() returns true (TreeMap$ValueIterator with TreeMap size=1)
* Column c = DeletedColumn
* counter.count() -> c.isLive returns false (c is DeletedColumn)
* counter.live() = 0
* iter.hasNext() returns false
* Math.min(0, toDiscard==1) returns 0

<- AbstractQueryPager.discardFirst(List<Row>):
* discarded = 0;
* count = newCf.getColumnCount() = 0;

-> assert discarded == 1 throws AssertionError