[CASSANDRA-6586] Cassandra touches all columns on CQL3 select

• Type: Improvement
• Status: Resolved
• Priority: Low
• Resolution: Duplicate
• Assignee: Unassigned
• Reporter: Jan Chochol
• Created: 15/Jan/14 09:11
• Updated: 16/Apr/19 09:31
• Resolved: 14/Mar/14 04:07

Description

It seems that Cassandra is checking (garbage collecting) all columns of all returned rows, despite the fact that not all columns are requested.

Example:

• use following script to fill Cassandra with test data:

  perl -e "print(\"DROP KEYSPACE t;\nCREATE KEYSPACE t WITH replication = {'class': 'SimpleStrategy', 'replication_factor' : 1};\nuse t;\nCREATE TABLE t (a varchar PRIMARY KEY, b varchar, c varchar, d varchar);\nCREATE INDEX t_b ON t (b);\nCREATE INDEX t_c ON t (c);\nCREATE INDEX t_d ON t (d);\n\");\$max = 200; for(\$i = 0; \$i < \$max; \$i++) { \$j = int(\$i * 10 / \$max); \$k = int(\$i * 100 / \$max); print(\"INSERT INTO t (a, b, c, d) VALUES ('a\$i', 'b\$j', 'c\$k', 'd\$i');\n\")}\n" | cqlsh
  

• turn on ALL logging for Cassandra
• issue this query:

  select a from t where c = 'c1';
  

This is result:

[root@jch3-devel:~/c4] cqlsh --no-color
Connected to C4 Cluster Single at localhost:9160.
[cqlsh 3.1.7 | Cassandra 1.2.11-SNAPSHOT | CQL spec 3.0.0 | Thrift protocol 19.36.1]
Use HELP for help.
cqlsh> use t;
cqlsh:t> select a from t where c = 'c1';

 a
----
 a3
 a2

From Cassandra log:

2014-01-15 09:14:56.663+0100 [Thrift:1] [TRACE] QueryProcessor.java(125) org.apache.cassandra.cql3.QueryProcessor: component=c4 Process org.apache.cassandra.cql3.statements.SelectStatement@614b3189 @CL.ONE
2014-01-15 09:14:56.810+0100 [Thrift:1] [TRACE] ReadCallback.java(67) org.apache.cassandra.service.ReadCallback: component=c4 Blockfor is 1; setting up requests to /127.0.0.1
2014-01-15 09:14:56.816+0100 [ReadStage:2] [DEBUG] CompositesSearcher.java(112) org.apache.cassandra.db.index.composites.CompositesSearcher: component=c4 Most-selective indexed predicate is 't.c EQ c1'
2014-01-15 09:14:56.817+0100 [ReadStage:2] [TRACE] ColumnFamilyStore.java(1493) org.apache.cassandra.db.ColumnFamilyStore: component=c4 Filtering org.apache.cassandra.db.index.composites.CompositesSearcher$1@e15911 for rows matching org.apache.cassandra.db.filter.ExtendedFilter$FilterWithCompositeClauses@4a9e6b8a
2014-01-15 09:14:56.817+0100 [ReadStage:2] [TRACE] CompositesSearcher.java(237) org.apache.cassandra.db.index.composites.CompositesSearcher: component=c4 Scanning index 't.c EQ c1' starting with 
2014-01-15 09:14:56.820+0100 [ReadStage:2] [TRACE] SSTableReader.java(776) org.apache.cassandra.io.sstable.SSTableReader: component=c4 Adding cache entry for KeyCacheKey(/mnt/ebs/cassandra/data/t/t/t-t.t_c-ic-1, 6331) -> org.apache.cassandra.db.RowIndexEntry@66a6574b
2014-01-15 09:14:56.821+0100 [ReadStage:2] [TRACE] SliceQueryFilter.java(164) org.apache.cassandra.db.filter.SliceQueryFilter: component=c4 collecting 0 of 10000: 6133:false:0@1389773577394000
2014-01-15 09:14:56.821+0100 [ReadStage:2] [TRACE] SliceQueryFilter.java(164) org.apache.cassandra.db.filter.SliceQueryFilter: component=c4 collecting 1 of 10000: 6132:false:0@1389773577391000
2014-01-15 09:14:56.822+0100 [ReadStage:2] [TRACE] CompositesSearcher.java(313) org.apache.cassandra.db.index.composites.CompositesSearcher: component=c4 Adding index hit to current row for 6133
2014-01-15 09:14:56.825+0100 [ReadStage:2] [TRACE] SSTableReader.java(776) org.apache.cassandra.io.sstable.SSTableReader: component=c4 Adding cache entry for KeyCacheKey(/mnt/ebs/cassandra/data/t/t/t-t-ic-1, 6133) -> org.apache.cassandra.db.RowIndexEntry@32ad3193
2014-01-15 09:14:56.826+0100 [ReadStage:2] [TRACE] SliceQueryFilter.java(164) org.apache.cassandra.db.filter.SliceQueryFilter: component=c4 collecting 0 of 2147483647: :false:0@1389773577394000
2014-01-15 09:14:56.826+0100 [ReadStage:2] [TRACE] SliceQueryFilter.java(164) org.apache.cassandra.db.filter.SliceQueryFilter: component=c4 collecting 1 of 2147483647: b:false:2@1389773577394000
2014-01-15 09:14:56.826+0100 [ReadStage:2] [TRACE] SliceQueryFilter.java(164) org.apache.cassandra.db.filter.SliceQueryFilter: component=c4 collecting 1 of 2147483647: c:false:2@1389773577394000
2014-01-15 09:14:56.826+0100 [ReadStage:2] [TRACE] SliceQueryFilter.java(164) org.apache.cassandra.db.filter.SliceQueryFilter: component=c4 collecting 1 of 2147483647: d:false:2@1389773577394000
2014-01-15 09:14:56.828+0100 [ReadStage:2] [TRACE] CompositesSearcher.java(313) org.apache.cassandra.db.index.composites.CompositesSearcher: component=c4 Adding index hit to current row for 6132
2014-01-15 09:14:56.828+0100 [ReadStage:2] [TRACE] SSTableReader.java(776) org.apache.cassandra.io.sstable.SSTableReader: component=c4 Adding cache entry for KeyCacheKey(/mnt/ebs/cassandra/data/t/t/t-t-ic-1, 6132) -> org.apache.cassandra.db.RowIndexEntry@87d66d5
2014-01-15 09:14:56.829+0100 [ReadStage:2] [TRACE] SliceQueryFilter.java(164) org.apache.cassandra.db.filter.SliceQueryFilter: component=c4 collecting 0 of 2147483647: :false:0@1389773577391000
2014-01-15 09:14:56.829+0100 [ReadStage:2] [TRACE] SliceQueryFilter.java(164) org.apache.cassandra.db.filter.SliceQueryFilter: component=c4 collecting 1 of 2147483647: b:false:2@1389773577391000
2014-01-15 09:14:56.829+0100 [ReadStage:2] [TRACE] SliceQueryFilter.java(164) org.apache.cassandra.db.filter.SliceQueryFilter: component=c4 collecting 1 of 2147483647: c:false:2@1389773577391000
2014-01-15 09:14:56.829+0100 [ReadStage:2] [TRACE] SliceQueryFilter.java(164) org.apache.cassandra.db.filter.SliceQueryFilter: component=c4 collecting 1 of 2147483647: d:false:2@1389773577391000
2014-01-15 09:14:56.829+0100 [ReadStage:2] [TRACE] CompositesSearcher.java(232) org.apache.cassandra.db.index.composites.CompositesSearcher: component=c4 Read only 2 (< 10000) last page through, must be done
2014-01-15 09:14:56.829+0100 [ReadStage:2] [TRACE] CompositesSearcher.java(232) org.apache.cassandra.db.index.composites.CompositesSearcher: component=c4 Read only 2 (< 10000) last page through, must be done
2014-01-15 09:14:56.830+0100 [Thrift:1] [DEBUG] Tracing.java(169) org.apache.cassandra.tracing.Tracing: component=c4 request complete

Note that Cassandra is checking all columns (a, b, c and d), even we requested only column a.
Things became really nasty, when using lots of columns, or bigger collections (yes - each member of collection is checked).

This is quite counter intuitive behaviour, as all Cassandra guides said, that using wide rows should not affect performance, but in CQL3 they create big performance bottleneck.

Issue Links

• duplicates: CASSANDRA-6588 Add a 'NO EMPTY RESULTS' filter to SELECT
• relates to: CASSANDRA-6588 Add a 'NO EMPTY RESULTS' filter to SELECT