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Save and restore query plans in PostgreSQL

Rationale

sr_plan looks like Oracle Outline system. It can be used to lock the execution plan. It is necessary if you do not trust the planner or able to form a better plan.

Build and install

make USE_PGXS=1
make USE_PGXS=1 install

and modify your postgres config:

shared_preload_libraries = 'sr_plan'

Usage

Install the extension in your database:

CREATE EXTENSION sr_plan;

If you want to save the query plan is necessary to set the variable:

set sr_plan.write_mode = true;

Now plans for all subsequent queries will be stored in the table sr_plans. Don't forget that all queries will be stored including duplicates.

Make an example query:

select query_hash from sr_plans where query_hash=10;

Disable saving the plan for the query:

set sr_plan.write_mode = false;

Enable it:

update sr_plans set enable=true;

After that, the plan for the query will be taken from the sr_plans.

In addition sr plan allows you to save a parameterized query plan. In this case, we have some constants in the query are not essential. For the parameters we use a special function _p (anyelement) example:

select query_hash from sr_plans where query_hash=1000+_p(10);

If we keep the plan for the query and enable it to be used also for the following queries:

select query_hash from sr_plans where query_hash=1000+_p(11);
select query_hash from sr_plans where query_hash=1000+_p(-5);

EXPLAIN for saved plans

It is possible to see saved plans by using show_plan function. It requires knowing query hash which could be fetched from sr_plans table.

Examples:

Show enabled plan for query hash:

SELECT show_plan(1);
                  show_plan                   
----------------------------------------------
 ("Seq Scan on public.explain_test")
 ("  Output: test_attr1, test_attr2")
 ("  Filter: (explain_test.test_attr1 = 10)")
(3 rows)

Get second saved plan by using index parameter (ignores enable attribute):

SELECT show_plan(1, index := 2);
                  show_plan                   
----------------------------------------------
 ("Seq Scan on public.explain_test")
 ("  Output: test_attr1, test_attr2")
 ("  Filter: (explain_test.test_attr1 = 10)")
(3 rows)

Use another output format (supported formats are json, text, xml, yaml):

SELECT show_plan(1, format := 'json');
                      show_plan                       
------------------------------------------------------
 ("[                                                 +
   {                                                 +
     ""Plan"": {                                     +
       ""Node Type"": ""Seq Scan"",                  +
       ""Parallel Aware"": false,                    +
       ""Relation Name"": ""explain_test"",          +
       ""Schema"": ""public"",                       +
       ""Alias"": ""explain_test"",                  +
       ""Output"": [""test_attr1"", ""test_attr2""], +
       ""Filter"": ""(explain_test.test_attr1 = 10)""+
     }                                               +
   }                                                 +
 ]")
(1 row)

pg_stat_statements integration

sr_plans table contains query_id columns which could be used to make joins with pg_stat_statements tables and views.

Note: in shared_preload_libraries list pg_stat_statements should be specified after sr_plan.