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https://issues.apache.org/jira/browse/DERBY-6216?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13696697#comment-13696697
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Knut Anders Hatlen commented on DERBY-6216:
-------------------------------------------
I like that the patch introduces a local flag so that the result set doesn't
have to go through the LCC and check if xplain is enabled for every row it
looks at. It might be a problem that the flag is set in the constructor,
though. The constructor is only called the first time a PreparedStatement is
executed, so it may cause some surprises if the same statement is used both
with xplain mode enabled and disabled.
Example ij session:
ij version 10.11
ij> connect 'jdbc:derby:memory:db;create=true';
ij> prepare ps1 as 'values ''Compiled before xplain mode was enabled''';
ij> execute ps1;
1
---------------------------------------
Compiled before xplain mode was enabled
1 row selected
ij> call syscs_util.syscs_set_runtimestatistics(1);
0 rows inserted/updated/deleted
ij> call syscs_util.syscs_set_xplain_schema('STATS');
0 rows inserted/updated/deleted
ij> call syscs_util.syscs_set_xplain_mode(1);
0 rows inserted/updated/deleted
ij> execute ps1;
1
---------------------------------------
0 rows selected
ij> prepare ps2 as 'values ''Compiled when xplain mode was enabled''';
ij> execute ps2;
1
-------------------------------------
0 rows selected
ij> call syscs_util.syscs_set_runtimestatistics(0);
0 rows inserted/updated/deleted
ij> call syscs_util.syscs_set_xplain_schema('');
0 rows inserted/updated/deleted
ij> call syscs_util.syscs_set_xplain_mode(0);
0 rows inserted/updated/deleted
ij> execute ps1;
1
---------------------------------------
Compiled before xplain mode was enabled
1 row selected
ij> execute ps2;
1
-------------------------------------
0 rows selected
ij>
The statement that was compiled before xplain was enabled actually behaved as
desired. It returned results if xplain mode was disabled, and returned empty
results if it was enabled. I think this statement works because
BasicNoPutResultSetImpl.getNextRow() still checks the LCC instead of the local
flag. (Note: this was in getNextRow(), not in getNextRowCore().)
The statement that was compiled when xplain was enabled, on the other hand,
continued returning empty results even after xplain had been disabled. I think
it would be more reasonable if it had returned results after disabling xplain
mode.
I think it might work better if the flag is initialized in open() instead of
the constructor.
> XPLAIN feature does not work and gives ERROR XCL16: ResultSet not open
> ----------------------------------------------------------------------
>
> Key: DERBY-6216
> URL: https://issues.apache.org/jira/browse/DERBY-6216
> Project: Derby
> Issue Type: Bug
> Components: Tools
> Affects Versions: 10.6.2.3, 10.7.1.4, 10.8.3.1, 10.9.1.0
> Environment: Solaris 10, MacOS X 10.6, Windows XP
> Reporter: Brett Bergquist
> Assignee: Bryan Pendleton
> Labels: derby_triage10_11, features
> Attachments: addTests.diff, emptyRS.diff, xplain-only.diff
>
>
> I am running Derby 10.9.1.0 and am trying to figure out a performance
> problem with query taking too long. I don’t actually want to run the query
> so from the documentation, I try:
> call syscs_util.syscs_set_runtimestatistics(1);
> call syscs_util.syscs_set_xplain_schema('STATS');
> call syscs_util.syscs_set_xplain_mode(1);
> --- my query here
> call syscs_util.syscs_set_runtimestatistics(0);
> call syscs_util.syscs_set_xplain_schema('');
> call syscs_util.syscs_set_xplain_mode(0);
> When it runs my query, I get:
> ERROR XCL16: ResultSet not open. Operation 'getMetaData' not permitted.
> Verify that autocommit is OFF.
> I try the following with IJ just to see:
> call syscs_util.syscs_set_runtimestatistics(1);
> call syscs_util.syscs_set_xplain_schema('STATS');
> call syscs_util.syscs_set_xplain_mode(1);
> select sql_text from syscs_diag.transaction_table where status != 'IDLE';
> call syscs_util.syscs_set_runtimestatistics(0);
> call syscs_util.syscs_set_xplain_schema('');
> call syscs_util.syscs_set_xplain_mode(0);
> Note I have tried this with real tables as apposed to the
> syscs_diag.transaction_table with the same result. The only reason that I
> posted this with this table is that it is a table that is accessible for any
> database.
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