How you use the XPLAIN style

Overview

XPLAIN style is an enhanced form of RUNTIMESTATISTICS processing which preserves captured statistics information in database tables. Once the statistics have been collected and saved in the tables, they can be queried for analysis purposes.

Note that Derby spells XPLAIN without the initial 'E'. This is done to help distinguish the Derby functionality from the explain functionality that you might be familiar with from commercial DBMS products. The current XPLAIN implementation is optimized for ad-hoc queries and tool support. Furthermore, the explain data is quite extensive to analyze. Derby tries to implement a compromise between detailed explain information which is almost unreadable by human users and which has to be evaluated with the help of a tool, versus a compact version of explain data which is only applicable for rough investigations but is still browseable by human users. We feel that the information in the XPLAIN system tables is sufficiently detailed to be powerful, but still simple enough to provide useful information to ad-hoc querying during interactive use.

To use XPLAIN style, first turn on RUNTIMESTATISTICS using the SYSCS_UTIL.SYSCS_SET_RUNTIMESTATISTICS() system procedure.

Optionally, turn on statistics timing using the SYSCS_UTIL.SYSCS_SET_STATISTICS_TIMING system procedure. If you do not turn on statistics timing, you will see the statement execution plan only, and not the timing information.

Next, to activate XPLAIN style, use the SYSCS_UTIL.SYSCS_SET_XPLAIN_SCHEMA() system procedure:

-- turn on RUNTIMESTATISTICS for connection:
CALL SYSCS_UTIL.SYSCS_SET_RUNTIMESTATISTICS(1);
CALL SYSCS_UTIL.SYSCS_SET_STATISTICS_TIMING(1);
-- Indicate that statistics information should be captured into
-- database tables in the MYSCHEMA schema:
CALL SYSCS_UTIL.SYSCS_SET_XPLAIN_SCHEMA('MYSCHEMA');
--execute queries, step through result sets, perform application processing...
CALL SYSCS_UTIL.SYSCS_SET_RUNTIMESTATISTICS(0);

Although the syntax is different, the basic steps for working with XPLAIN style are the same in a Java program.

XPLAIN style is further refined by the use of XPLAIN-only mode. By default, XPLAIN-only mode is off, which means that statements are compiled and executed normally. When XPLAIN-only mode is on, statements are compiled, but not executed. This is useful for investigating what query plan has been selected for a statement, without actually executing the statement. To activate XPLAIN-only mode, use the SYSCS_UTIL.SYSCS_SET_XPLAIN_MODE() system procedure:

    call SYSCS_UTIL.SYSCS_SET_XPLAIN_MODE(1);

Examples

Some examples of usage follow.

Retrieve the text of statements which were captured, in order by the time when the statistics were captured:

    select stmt_text, xplain_time from myschema.sysxplain_statements
    order by xplain_time

Retrieve the text of statements which were captured, showing the statements which took the longest time to execute first:

    select s.stmt_text, st.execute_time from myschema.sysxplain_statements s,
           myschema.sysxplain_statement_timings st
    where s.timing_id = st.timing_id
    order by st.execute_time desc

Show the statements that were executed, together with the result sets that each statement required:

    select st.stmt_text, rs.op_identifier 
    from myschema.sysxplain_statements st
    join myschema.sysxplain_resultsets rs
         on st.stmt_id = rs.stmt_id

Find statements which resulted in an external sort being performed:

    select s.stmt_text, s.stmt_id, rs.op_identifier, srt.no_input_rows
    from myschema.sysxplain_sort_props srt,
         myschema.sysxplain_resultsets rs,
         myschema.sysxplain_statements s
    where rs.stmt_id = s.stmt_id and rs.sort_rs_id = srt.sort_rs_id
          and srt.sort_type = 'EX'

Show statements which resulted in a sort, sorted by the number of rows which were sorted by that statement.

    select s.stmt_text, s.stmt_id, rs.op_identifier, srt.no_input_rows
    from myschema.sysxplain_sort_props srt,
         myschema.sysxplain_resultsets rs,
         myschema.sysxplain_statements s
    where rs.stmt_id = s.stmt_id and rs.sort_rs_id = srt.sort_rs_id
    order by srt.no_input_rows desc

Find statements which resulted in a tablescan of the COUNTRIES table, and display the number of pages and rows that were visited by each scan:

    select st.stmt_text, sp.no_visited_pages, sp.no_visited_rows 
    from myschema.sysxplain_scan_props sp, 
         myschema.sysxplain_resultsets rs, 
         myschema.sysxplain_statements st 
    where st.stmt_id = rs.stmt_id and 
          rs.scan_rs_id = sp.scan_rs_id and 
          rs.op_identifier = 'TABLESCAN' and 
          sp.scan_object_name = 'COUNTRIES'