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https://issues.apache.org/jira/browse/CASSANDRA-6586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13872555#comment-13872555
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Donald Smith commented on CASSANDRA-6586:
-----------------------------------------
To clarify (and correct me if I'm wrong), this ticket does *not* imply that all
physical thrift columns (cells) of a physical thrift row (partition) are read
when you do a CQL select on a CQL primary key. It just means that all columns
mentioned in the CQL primary key are read. There's still a lot of confusion
between thrift terminology and CQL terminology.
You can still have wide rows and you still can avoid reading all the (physical
thrift) columns of that row.
People are still confused by terminology and this scares them unnecessarily.
http://www.datastax.com/dev/blog/does-cql-support-dynamic-columns-wide-rows
explains the terminology to use, but it's still unclear.
> Cassandra touches all columns on CQL3 select
> --------------------------------------------
>
> Key: CASSANDRA-6586
> URL: https://issues.apache.org/jira/browse/CASSANDRA-6586
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Jan Chochol
> Priority: Minor
>
> 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:
> {noformat}
> 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
> {noformat}
> * turn on {{ALL}} logging for Cassandra
> * issue this query:
> {noformat}
> select a from t where c = 'c1';
> {noformat}
> This is result:
> {noformat}
> [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
> {noformat}
> From Cassandra log:
> {noformat}
> 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
> {noformat}
> 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.
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