Monday, 20 October 2014

When to ANALYZE in Greenplum and HAWQ?

When to ANALYZE in Greenplum and HAWQ?

Greenplum and HAWQ will perform an ANALYZE automatically for you so the query optimizer will have good statistics and build a good plan. Greenplum and HAWQ also allow you to configure this if needed with gp_autostats_mode and gp_autostats_on_change_threshold.
gp_autostats_mode
This specifies when an automatic ANALYZE should take place. The values for this configuration parameter can be:
none
on_change
on_no_stats
on_no_stats
The default is on_no_stats and the database will automatically perform an ANALYZE for you. If you INSERT data into an empty table (new table or a table you just did a TRUNCATE on), the database will automatically gather statistics with an ANALYZE.
Typically in an Analytics Data Warehouse, you will insert data once and then read it many times. So the default of on_no_stats will automatically give you statistics as needed with no additional work from you.
none
Self explanatory. You have to execute ANALYZE if you want statistics.
on_change
With this setting, if perform an INSERT, UPDATE, or DELETE that exceeds the gp_autostats_on_change_threshold value, then an automatic ANALYZE will happen. Note for HAWQ, you can only INSERT.
In Greenplum and in the rare case where you are doing a DELETE or UPDATE to a table, you will need to execute an ANALYZE statement. Or, you can set the database to use on_change and the ANALYZE will happen automatically.
gp_autostats_on_change_threshold
This is only relevant for on_change and it is the number of rows that need to change before an automatic ANALYZE will happen. The default is over 2 billion rows so if you really want to use on_change, then you will also need to reduce this configuration parameter to something more realistic.
Examples
Example 1 – The default
gp_autostats_mode = on_no_stats
gp_autostats_on_change_threshold = 2147483647
CREATE TABLE my_table AS 
SELECT state, count(*) AS counter
FROM customer
GROUP BY state
DISTRIBUTED BY (state);
--Statistics will be gathered automatically.
CREATE TABLE my_table 
(state varchar(100),
 counter int)
DISTRIBUTED BY (state);

INSERT INTO my_table
SELECT state, count(*) AS counter
FROM customer
GROUP BY state;
--Statistics will be gathered automatically.
TRUNCATE my_table;

INSERT INTO my_table
SELECT state, count(*) AS counter
FROM customer
GROUP BY state;
--Statistics will be gathered automatically.
TRUNCATE my_table;

INSERT INTO my_table
VALUES ('dummy', 0);
--Statistics will be gathered automatically.

INSERT INTO my_table
SELECT state, count(*) AS counter
FROM customer
GROUP BY state;
--Statistics won't be updated.  The planner will think the table has only 1 row.

ANALYZE my_table;
--Statistics manually gathered and correctly shows the correct number of rows in the table.
Example 2 – Using on_change
gp_autostats_mode = on_change
gp_autostats_on_change_threshold = 1000000
CREATE TABLE my_table AS 
SELECT state, count(*) AS counter
FROM customer
GROUP BY state
DISTRIBUTED BY (state);
--Statistics will be gathered automatically only if the number of rows is 1M or more.
CREATE TABLE my_table 
(state varchar(100),
 counter int)
DISTRIBUTED BY (state);

INSERT INTO my_table
SELECT state, count(*) AS counter
FROM customer
GROUP BY state;
--Statistics will be gathered automatically only if the number of rows is 1M or more.
TRUNCATE my_table;

INSERT INTO my_table
SELECT state, count(*) AS counter
FROM customer
GROUP BY state;
--Statistics will be gathered automatically only if the number of rows is 1M or more.
TRUNCATE my_table;

INSERT INTO my_table
VALUES ('dummy', 0);
--Statistics will not be gathered automatically.

INSERT INTO my_table
SELECT state, count(*) AS counter
FROM customer
GROUP BY state;
--Statistics will be gathered automatically only if the number of rows is 1M or more.
Checking the Statistics
And here are a couple of queries you can use to see the statics information.
SELECT c.oid, c.relpages, c.reltuples 
FROM pg_class c 
JOIN pg_namespace n ON c.relnamespace = n.oid 
WHERE n.nspname = 'public' 
AND c.relname = 'my_table';
--using the oid from the previous query
SELECT * FROM pg_statistic WHERE starelid = 298610;
Summary
Greenplum and HAWQ automatically gather statistics for you in most cases. If you are doing lots of DML activity, you can change the configuration to still automatically gather statistics for you too. These automatic settings make life easier for DBAs, Developers, and Analysts but still give you the flexibility to configure it in the best way for your environment.
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