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Franck Pachot for YugabyteDB

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pg_stat_statements with pg_hint_plan

I got this question when presenting pg_hint_plan at the PgDay Austria (I'll present it again, online, at postgresconf.org): does the queryid in pg_stat_statements change when hints are added to the query?

The short answer is: no, it doesn't change, the queryid is calculated from the post-parse-analysis tree, ignoring the comments, and this includes hints.

But this also answers another question: do we see the injected hints in the query text? Here is a little demo to show that, even when the query is modified from the hint_plan.hints table, the queryid and query text is the same are the same as without hints.

I create the tables from my demo:

create table a (n int primary key, x int);
create table b (n int primary key, x int);
create table c (n int primary key, x int);
create index b_index on b(x) include (n);
select pg_stat_statements_reset();
select * from c "🍒" natural join b "🍌" natural join a "🍏" ;
select queryid, calls, rows, query from pg_stat_statements;
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I check the queryid from pg_stat_statements:

yugabyte=# select * from c "🍒" natural join b "🍌" natural join a "🍏" ;
select queryid, calls, rows, query from pg_stat_statements;LOG:  statement: select * from c "🍒" natural join b "🍌" natural join a "🍏" ;
 n | x
---+---
(0 rows)

yugabyte=# select queryid, calls, rows, query from pg_stat_statements;

       queryid       | calls | rows |                           query
---------------------+-------+------+-----------------------------------------------------------
 6990186059047281266 |     1 |    1 | select pg_stat_statements_reset()
 6386600050796028530 |     1 |    0 | select * from c "🍒" natural join b "🍌" natural join a "🍏"
(2 rows)
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Now with the hints from the hint_plan table:

grant yb_extension to yugabyte;
create extension pg_hint_plan;
set pg_hint_plan.enable_hint_table=on;
insert into hint_plan.hints(norm_query_string, application_name, hints) values (
$$select * from c "🍒" natural join b "🍌" natural join a "🍏" ;$$,
$$$$,
$hints$Leading( ( ("🍏" "🍌") "🍒" ) ) HashJoin( "🍏" "🍌" ) HashJoin("🍏" "🍌" "🍒") SeqScan("🍏") IndexOnlyScan("🍌") SeqScan("🍒")$hints$
);
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I see the same statement and and queryid:

yugabyte=# select * from c "🍒" natural join b "🍌" natural join a "🍏" ;
select queryid, calls, rows, query from pg_stat_statements; n | x
---+---
(0 rows)

yugabyte=# select queryid, calls, rows, query from pg_stat_statements;
       queryid        | calls | rows |                                                                              query
----------------------+-------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------
 -4857947532354760446 |     3 |    1 | SELECT hints   FROM hint_plan.hints  WHERE norm_query_string = $1    AND ( application_name = $2     OR application_name = '' )  ORDER BY application_name DESC
  6990186059047281266 |     1 |    1 | select pg_stat_statements_reset()
  6386600050796028530 |     1 |    0 | select * from c "🍒" natural join b "🍌" natural join a "🍏"
(3 rows)
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Then, how can I know that the hint was used? I can enable the verbose logs:

yugabyte=# set pg_hint_plan.debug_print=verbose;
SET
yugabyte=# set client_min_messages = log;
SET
yugabyte=# select pg_stat_statements_reset();
LOG:  statement: select pg_stat_statements_reset();
LOG:  pg_hint_plan[qno=0x1a]: no match found in table:  application name = "psql", normalized_query="select pg_stat_statements_reset();"
LOG:  hints in comment="(none)", query="select pg_stat_statements_reset();", debug_query_string="select pg_stat_statements_reset();"
LOG:  pg_hint_plan[qno=0x1c]: no match found in table:  application name = "psql", normalized_query="select pg_stat_statements_reset();"
LOG:  hints in comment="(none)", query="select pg_stat_statements_reset();", debug_query_string="select pg_stat_statements_reset();"
LOG:  pg_hint_plan[qno=0x1a]: planner: no valid hint
 pg_stat_statements_reset
--------------------------

(1 row)

yugabyte=# select * from c "🍒" natural join b "🍌" natural join a "🍏" ;
select queryid, calls, rows, query from pg_stat_statements;LOG:  statement: select * from c "🍒" natural join b "🍌" natural join a "🍏" ;
LOG:  pg_hint_plan[qno=0x1e]: post_parse_analyze_hook: hints from table: "Leading( ( ("🍏" "🍌") "🍒" ) ) HashJoin( "🍏" "🍌" ) HashJoin("🍏" "🍌" "🍒") SeqScan("🍏") IndexOnlyScan("🍌") SeqScan("🍒")": normalized_query="select * from c "🍒" natural join b "🍌" natural join a "🍏" ;", application name ="psql"
LOG:  pg_hint_plan[qno=0x1c]: planner
LOG:  pg_hint_plan[qno=0x1c]: setup_hint_enforcement index deletion: relation=16659(c), inhparent=0, current_hint_state=0xe094648, hint_inhibit_level=0, scanmask=0x1
LOG:  pg_hint_plan[qno=0x1c]: setup_hint_enforcement index deletion: relation=16654(b), inhparent=0, current_hint_state=0xe094648, hint_inhibit_level=0, scanmask=0x12
LOG:  pg_hint_plan[qno=0x1c]: setup_hint_enforcement index deletion: relation=16649(a), inhparent=0, current_hint_state=0xe094648, hint_inhibit_level=0, scanmask=0x1
LOG:  pg_hint_plan[qno=0x1c]: HintStateDump: {used hints:IndexOnlyScan(🍌)SeqScan(🍏)SeqScan(🍒)HashJoin(🍌 🍏)HashJoin(🍌 🍏 🍒)Leading(((🍏 🍌) 🍒))}, {not used hints:(none)}, {duplicate hints:(none)}, {error hints:(none)}
 n | x
---+---
(0 rows)

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When pg_hint_plan.debug_print=verbose the hints that have been injected by pg_hint_plan.enable_hint_table=on are visible in the HintStateDump.

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