mirror of
https://github.com/apache/impala.git
synced 2026-01-26 21:02:23 -05:00
f3820e6205
For history and tracking purposes, there are many instances of rev="CDH-1234" for various CDH- JIRA numbers. This produces no visible output, it's just FYI for the person editing the source. Removing all these now from the upstream doc source, so as not to have "CDH" all through the doc source files. Change-Id: I29089e5a31cd72e876b2ccb8375d1c10693c6aba Reviewed-on: http://gerrit.cloudera.org:8080/6349 Reviewed-by: Ambreen Kazi <ambreen.kazi@cloudera.com> Reviewed-by: John Russell <jrussell@cloudera.com> Tested-by: Impala Public Jenkins
283 lines
14 KiB
XML
283 lines
14 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!--
|
|
Licensed to the Apache Software Foundation (ASF) under one
|
|
or more contributor license agreements. See the NOTICE file
|
|
distributed with this work for additional information
|
|
regarding copyright ownership. The ASF licenses this file
|
|
to you under the Apache License, Version 2.0 (the
|
|
"License"); you may not use this file except in compliance
|
|
with the License. You may obtain a copy of the License at
|
|
|
|
http://www.apache.org/licenses/LICENSE-2.0
|
|
|
|
Unless required by applicable law or agreed to in writing,
|
|
software distributed under the License is distributed on an
|
|
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
|
|
KIND, either express or implied. See the License for the
|
|
specific language governing permissions and limitations
|
|
under the License.
|
|
-->
|
|
<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
|
|
<concept id="explain">
|
|
|
|
<title>EXPLAIN Statement</title>
|
|
<titlealts audience="PDF"><navtitle>EXPLAIN</navtitle></titlealts>
|
|
<prolog>
|
|
<metadata>
|
|
<data name="Category" value="Impala"/>
|
|
<data name="Category" value="SQL"/>
|
|
<data name="Category" value="Querying"/>
|
|
<data name="Category" value="Reports"/>
|
|
<data name="Category" value="Planning"/>
|
|
<data name="Category" value="Performance"/>
|
|
<data name="Category" value="Troubleshooting"/>
|
|
<data name="Category" value="Administrators"/>
|
|
<data name="Category" value="Developers"/>
|
|
<data name="Category" value="Data Analysts"/>
|
|
</metadata>
|
|
</prolog>
|
|
|
|
<conbody>
|
|
|
|
<p>
|
|
<indexterm audience="hidden">EXPLAIN statement</indexterm>
|
|
Returns the execution plan for a statement, showing the low-level mechanisms that Impala will use to read the
|
|
data, divide the work among nodes in the cluster, and transmit intermediate and final results across the
|
|
network. Use <codeph>explain</codeph> followed by a complete <codeph>SELECT</codeph> query. For example:
|
|
</p>
|
|
|
|
<p conref="../shared/impala_common.xml#common/syntax_blurb"/>
|
|
|
|
<codeblock>EXPLAIN { <varname>select_query</varname> | <varname>ctas_stmt</varname> | <varname>insert_stmt</varname> }
|
|
</codeblock>
|
|
|
|
<p>
|
|
The <varname>select_query</varname> is a <codeph>SELECT</codeph> statement, optionally prefixed by a
|
|
<codeph>WITH</codeph> clause. See <xref href="impala_select.xml#select"/> for details.
|
|
</p>
|
|
|
|
<p>
|
|
The <varname>insert_stmt</varname> is an <codeph>INSERT</codeph> statement that inserts into or overwrites an
|
|
existing table. It can use either the <codeph>INSERT ... SELECT</codeph> or <codeph>INSERT ...
|
|
VALUES</codeph> syntax. See <xref href="impala_insert.xml#insert"/> for details.
|
|
</p>
|
|
|
|
<p>
|
|
The <varname>ctas_stmt</varname> is a <codeph>CREATE TABLE</codeph> statement using the <codeph>AS
|
|
SELECT</codeph> clause, typically abbreviated as a <q>CTAS</q> operation. See
|
|
<xref href="impala_create_table.xml#create_table"/> for details.
|
|
</p>
|
|
|
|
<p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>
|
|
|
|
<p>
|
|
You can interpret the output to judge whether the query is performing efficiently, and adjust the query
|
|
and/or the schema if not. For example, you might change the tests in the <codeph>WHERE</codeph> clause, add
|
|
hints to make join operations more efficient, introduce subqueries, change the order of tables in a join, add
|
|
or change partitioning for a table, collect column statistics and/or table statistics in Hive, or any other
|
|
performance tuning steps.
|
|
</p>
|
|
|
|
<p>
|
|
The <codeph>EXPLAIN</codeph> output reminds you if table or column statistics are missing from any table
|
|
involved in the query. These statistics are important for optimizing queries involving large tables or
|
|
multi-table joins. See <xref href="impala_compute_stats.xml#compute_stats"/> for how to gather statistics,
|
|
and <xref href="impala_perf_stats.xml#perf_stats"/> for how to use this information for query tuning.
|
|
</p>
|
|
|
|
<p conref="../shared/impala_common.xml#common/explain_interpret"/>
|
|
|
|
<p>
|
|
If you come from a traditional database background and are not familiar with data warehousing, keep in mind
|
|
that Impala is optimized for full table scans across very large tables. The structure and distribution of
|
|
this data is typically not suitable for the kind of indexing and single-row lookups that are common in OLTP
|
|
environments. Seeing a query scan entirely through a large table is common, not necessarily an indication of
|
|
an inefficient query. Of course, if you can reduce the volume of scanned data by orders of magnitude, for
|
|
example by using a query that affects only certain partitions within a partitioned table, then you might be
|
|
able to optimize a query so that it executes in seconds rather than minutes.
|
|
</p>
|
|
|
|
<p>
|
|
For more information and examples to help you interpret <codeph>EXPLAIN</codeph> output, see
|
|
<xref href="impala_explain_plan.xml#perf_explain"/>.
|
|
</p>
|
|
|
|
<p rev="1.2">
|
|
<b>Extended EXPLAIN output:</b>
|
|
</p>
|
|
|
|
<p rev="1.2">
|
|
For performance tuning of complex queries, and capacity planning (such as using the admission control and
|
|
resource management features), you can enable more detailed and informative output for the
|
|
<codeph>EXPLAIN</codeph> statement. In the <cmdname>impala-shell</cmdname> interpreter, issue the command
|
|
<codeph>SET EXPLAIN_LEVEL=<varname>level</varname></codeph>, where <varname>level</varname> is an integer
|
|
from 0 to 3 or corresponding mnemonic values <codeph>minimal</codeph>, <codeph>standard</codeph>,
|
|
<codeph>extended</codeph>, or <codeph>verbose</codeph>.
|
|
</p>
|
|
|
|
<p rev="1.2">
|
|
When extended <codeph>EXPLAIN</codeph> output is enabled, <codeph>EXPLAIN</codeph> statements print
|
|
information about estimated memory requirements, minimum number of virtual cores, and so on.
|
|
<!--
|
|
that you can use to fine-tune the resource management options explained in <xref href="impala_resource_management.xml#rm_options"/>.
|
|
(The estimated memory requirements are intentionally on the high side, to allow a margin for error,
|
|
to avoid cancelling a query unnecessarily if you set the <codeph>MEM_LIMIT</codeph> option to the estimated memory figure.)
|
|
-->
|
|
</p>
|
|
|
|
<p>
|
|
See <xref href="impala_explain_level.xml#explain_level"/> for details and examples.
|
|
</p>
|
|
|
|
<p conref="../shared/impala_common.xml#common/example_blurb"/>
|
|
|
|
<p>
|
|
This example shows how the standard <codeph>EXPLAIN</codeph> output moves from the lowest (physical) level to
|
|
the higher (logical) levels. The query begins by scanning a certain amount of data; each node performs an
|
|
aggregation operation (evaluating <codeph>COUNT(*)</codeph>) on some subset of data that is local to that
|
|
node; the intermediate results are transmitted back to the coordinator node (labelled here as the
|
|
<codeph>EXCHANGE</codeph> node); lastly, the intermediate results are summed to display the final result.
|
|
</p>
|
|
|
|
<codeblock id="explain_plan_simple">[impalad-host:21000] > explain select count(*) from customer_address;
|
|
+----------------------------------------------------------+
|
|
| Explain String |
|
|
+----------------------------------------------------------+
|
|
| Estimated Per-Host Requirements: Memory=42.00MB VCores=1 |
|
|
| |
|
|
| 03:AGGREGATE [MERGE FINALIZE] |
|
|
| | output: sum(count(*)) |
|
|
| | |
|
|
| 02:EXCHANGE [PARTITION=UNPARTITIONED] |
|
|
| | |
|
|
| 01:AGGREGATE |
|
|
| | output: count(*) |
|
|
| | |
|
|
| 00:SCAN HDFS [default.customer_address] |
|
|
| partitions=1/1 size=5.25MB |
|
|
+----------------------------------------------------------+
|
|
</codeblock>
|
|
|
|
<p>
|
|
These examples show how the extended <codeph>EXPLAIN</codeph> output becomes more accurate and informative as
|
|
statistics are gathered by the <codeph>COMPUTE STATS</codeph> statement. Initially, much of the information
|
|
about data size and distribution is marked <q>unavailable</q>. Impala can determine the raw data size, but
|
|
not the number of rows or number of distinct values for each column without additional analysis. The
|
|
<codeph>COMPUTE STATS</codeph> statement performs this analysis, so a subsequent <codeph>EXPLAIN</codeph>
|
|
statement has additional information to use in deciding how to optimize the distributed query.
|
|
</p>
|
|
|
|
<!-- To do:
|
|
Re-run these examples with more substantial tables populated with data.
|
|
-->
|
|
|
|
<codeblock rev="1.2">[localhost:21000] > set explain_level=extended;
|
|
EXPLAIN_LEVEL set to extended
|
|
[localhost:21000] > explain select x from t1;
|
|
[localhost:21000] > explain select x from t1;
|
|
+----------------------------------------------------------+
|
|
| Explain String |
|
|
+----------------------------------------------------------+
|
|
| Estimated Per-Host Requirements: Memory=32.00MB VCores=1 |
|
|
| |
|
|
| 01:EXCHANGE [PARTITION=UNPARTITIONED] |
|
|
| | hosts=1 per-host-mem=unavailable |
|
|
<b>| | tuple-ids=0 row-size=4B cardinality=unavailable |</b>
|
|
| | |
|
|
| 00:SCAN HDFS [default.t2, PARTITION=RANDOM] |
|
|
| partitions=1/1 size=36B |
|
|
<b>| table stats: unavailable |</b>
|
|
<b>| column stats: unavailable |</b>
|
|
| hosts=1 per-host-mem=32.00MB |
|
|
<b>| tuple-ids=0 row-size=4B cardinality=unavailable |</b>
|
|
+----------------------------------------------------------+
|
|
</codeblock>
|
|
|
|
<codeblock rev="1.2">[localhost:21000] > compute stats t1;
|
|
+-----------------------------------------+
|
|
| summary |
|
|
+-----------------------------------------+
|
|
| Updated 1 partition(s) and 1 column(s). |
|
|
+-----------------------------------------+
|
|
[localhost:21000] > explain select x from t1;
|
|
+----------------------------------------------------------+
|
|
| Explain String |
|
|
+----------------------------------------------------------+
|
|
| Estimated Per-Host Requirements: Memory=64.00MB VCores=1 |
|
|
| |
|
|
| 01:EXCHANGE [PARTITION=UNPARTITIONED] |
|
|
| | hosts=1 per-host-mem=unavailable |
|
|
| | tuple-ids=0 row-size=4B cardinality=0 |
|
|
| | |
|
|
| 00:SCAN HDFS [default.t1, PARTITION=RANDOM] |
|
|
| partitions=1/1 size=36B |
|
|
<b>| table stats: 0 rows total |</b>
|
|
<b>| column stats: all |</b>
|
|
| hosts=1 per-host-mem=64.00MB |
|
|
<b>| tuple-ids=0 row-size=4B cardinality=0 |</b>
|
|
+----------------------------------------------------------+
|
|
</codeblock>
|
|
|
|
<p conref="../shared/impala_common.xml#common/security_blurb"/>
|
|
<p conref="../shared/impala_common.xml#common/redaction_yes"/>
|
|
|
|
<p conref="../shared/impala_common.xml#common/cancel_blurb_no"/>
|
|
|
|
<p conref="../shared/impala_common.xml#common/permissions_blurb"/>
|
|
<p rev="">
|
|
<!-- Doublecheck these details. Does EXPLAIN really need any permissions? -->
|
|
The user ID that the <cmdname>impalad</cmdname> daemon runs under,
|
|
typically the <codeph>impala</codeph> user, must have read
|
|
and execute permissions for all applicable directories in all source tables
|
|
for the query that is being explained.
|
|
(A <codeph>SELECT</codeph> operation could read files from multiple different HDFS directories
|
|
if the source table is partitioned.)
|
|
</p>
|
|
|
|
<p rev="kudu" conref="../shared/impala_common.xml#common/kudu_blurb"/>
|
|
<p>
|
|
The <codeph>EXPLAIN</codeph> statement displays equivalent plan
|
|
information for queries against Kudu tables as for queries
|
|
against HDFS-based tables.
|
|
</p>
|
|
|
|
<p>
|
|
To see which predicates Impala can <q>push down</q> to Kudu for
|
|
efficient evaluation, without transmitting unnecessary rows back
|
|
to Impala, look for the <codeph>kudu predicates</codeph> item in
|
|
the scan phase of the query. The label <codeph>kudu predicates</codeph>
|
|
indicates a condition that can be evaluated efficiently on the Kudu
|
|
side. The label <codeph>predicates</codeph> in a <codeph>SCAN KUDU</codeph>
|
|
node indicates a condition that is evaluated by Impala.
|
|
For example, in a table with primary key column <codeph>X</codeph>
|
|
and non-primary key column <codeph>Y</codeph>, you can see that
|
|
some operators in the <codeph>WHERE</codeph> clause are evaluated
|
|
immediately by Kudu and others are evaluated later by Impala:
|
|
<codeblock>
|
|
EXPLAIN SELECT x,y from kudu_table WHERE
|
|
x = 1 AND x NOT IN (2,3) AND y = 1
|
|
AND x IS NOT NULL AND x > 0;
|
|
+----------------
|
|
| Explain String
|
|
+----------------
|
|
...
|
|
| 00:SCAN KUDU [jrussell.hash_only]
|
|
| predicates: x IS NOT NULL, x NOT IN (2, 3)
|
|
| kudu predicates: x = 1, x > 0, y = 1
|
|
</codeblock>
|
|
Only binary predicates and <codeph>IN</codeph> predicates containing
|
|
literal values that exactly match the types in the Kudu table, and do not
|
|
require any casting, can be pushed to Kudu.
|
|
</p>
|
|
|
|
<p conref="../shared/impala_common.xml#common/related_info"/>
|
|
<p>
|
|
<xref href="impala_select.xml#select"/>,
|
|
<xref href="impala_insert.xml#insert"/>,
|
|
<xref href="impala_create_table.xml#create_table"/>,
|
|
<xref href="impala_explain_plan.xml#explain_plan"/>
|
|
</p>
|
|
|
|
</conbody>
|
|
</concept>
|