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https://issues.apache.org/jira/browse/HAWQ-800?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15326423#comment-15326423
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Ruilong Huo commented on HAWQ-800:
----------------------------------
Per investigation and discussion with Hubert and Ming, this is a bug introduced
in prepare statement (prepare either in udf or in sql statement) in hawq 2.0.
The root cause is that there are two phases in prepared statement: phase 1)
plan + metadata + datalocality is prepared and dispatched only once; phase 2)
every time the plan is executed, it use the same datalocality.
For example, table t (id int) have 1 tuple at beginning, if we do below for
loop with "insert into t select * from t" as prepared statement, it gives below
result:
{noformat}
phase
content in table t hawq 2.0
hawq 1.x
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
beginning i = 0
{1}
{1}
for i = 1 ~ 3: do "insert into t select * from t" i = 1
{1, 1} {1, 1}
i
= 2 {1, 1, 1}
{1, 1, 1, 1}
i
= 3 {1, 1, 1, 1}
{1, 1, 1, 1, 1, 1, 1, 1}
{noformat}
To be specific, every time it execute the "insert into select", it uses the
initial datalocality (i.e., initial tuple {1} in table t) hawq 2.0; while in
hawq 1.x, every segment read all its latest data and do the insert.
To keep the prepared statement feature, we propose below fix:
1. Separate the dispatching into two separate part: 1) plan+metadata, which is
the same as 1.x; 2) datalocality + resource_negotiator
2. Every time we dispatch prepared plan + metadata in 1) for execution, we
re-calculate the datalocality and re-do the resource negotiation, and then
dispatch it along with 1).
3. If the resource (i.e., vseg number) change in above step 2, we re-prepare
and then cache the new plan
PS: note that for prepared statement in UDF, we use a fixed number of vsegs;
while in prepared statement in SQL, data size change may incur change of
resource (i.e., vseg number) and thus the plan.
> Less tuple is inserted due to data locality information is not refreshed and
> dispatched in prepared statement
> -------------------------------------------------------------------------------------------------------------
>
> Key: HAWQ-800
> URL: https://issues.apache.org/jira/browse/HAWQ-800
> Project: Apache HAWQ
> Issue Type: Bug
> Components: Core, Dispatcher, Query Execution, Resource Manager
> Reporter: Ruilong Huo
> Assignee: Ruilong Huo
> Attachments: proba.out, proba.sql, proba_execute.out,
> proba_execute.sql
>
>
> In either explicit (SQL) or implicit (UDF) prepared statement, there is less
> tuple inserted if we run a prepared "insert into t select * from t" plan
> multiple times in a transaction.
> Below is a simple case to reproduce this issue. For a more complicated
> example, you may refer to attached proba_execute and proba.
> 1. There should be 8 tuples, however there is only 4 in hawq 2.0
> {noformat}
> drop table if exists t;
> DROP TABLE
> create table t (id int);
> CREATE TABLE
> insert into t values (1);
> INSERT 0 1
> CREATE OR REPLACE FUNCTION f_load()
> RETURNS TEXT
> LANGUAGE plpgsql
> AS
> $body$
> DECLARE
> l_rec RECORD;
> l_itm RECORD;
> BEGIN
> FOR l_rec IN ( SELECT generate_series(1, 3) AS id )
> LOOP
> INSERT INTO t SELECT * FROM t;
> END LOOP;
> RETURN 'done';
> END;
> $body$
> ;
> CREATE FUNCTION
> SELECT f_load();
> f_load
> --------
> done
> (1 row)
> SELECT * FROM t;
> id
> ----
> 1
> 1
> 1
> 1
> (4 rows)
> {noformat}
> 2. There are 8 tuples as expected in hawq 1.x
> {noformat}
> drop table if exists t;
> DROP TABLE
> create table t (id int);
> psql:temp.sql:3: NOTICE: Table doesn't have 'DISTRIBUTED BY' clause -- Using
> column named 'id' as the Greenplum Database data distribution key for this
> table.
> HINT: The 'DISTRIBUTED BY' clause determines the distribution of data. Make
> sure column(s) chosen are the optimal data distribution key to minimize skew.
> CREATE TABLE
> insert into t values (1);
> INSERT 0 1
> CREATE OR REPLACE FUNCTION f_load()
> RETURNS TEXT
> LANGUAGE plpgsql
> AS
> $body$
> DECLARE
> l_rec RECORD;
> l_itm RECORD;
> BEGIN
> FOR l_rec IN ( SELECT generate_series(1, 3) AS id )
> LOOP
> INSERT INTO t SELECT * FROM t;
> END LOOP;
> RETURN 'done';
> END;
> $body$
> ;
> CREATE FUNCTION
> SELECT f_load();
> f_load
> --------
> done
> (1 row)
> SELECT * FROM t;
> id
> ----
> 1
> 1
> 1
> 1
> 1
> 1
> 1
> 1
> (8 rows)
> {noformat}
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