Access Method Hints v12
The following hints influence how the optimizer accesses relations to create the result set.
| Hint | Description |
|---|---|
FULL(table) | Perform a full sequential scan on table. |
INDEX(table [ index ] [...]) | Use index on table to access the relation. |
NO_INDEX(table [ index ] [...]) | Do not use index on table to access the relation. |
In addition, the ALL_ROWS, FIRST_ROWS, and FIRST_ROWS(n) hints can be used.
Examples
The sample application does not have sufficient data to illustrate the effects of optimizer hints so the remainder of the examples in this section will use a banking database created by the pgbench application located in the Advanced Server bin subdirectory.
The following steps create a database named, bank, populated by the tables, pgbench_accounts, pgbench_branches, pgbench_tellers, and pgbench_history. The –s 20 option specifies a scaling factor of twenty, which results in the creation of twenty branches, each with 100,000 accounts, resulting in a total of 2,000,000 rows in the pgbench_accounts table and twenty rows in the pgbench_branches table. Ten tellers are assigned to each branch resulting in a total of 200 rows in the pgbench_tellers table.
The following initializes the pgbench application in the bank database.
createdb -U enterprisedb bank CREATE DATABASE pgbench -i -s 20 -U enterprisedb bank NOTICE: table "pgbench_history" does not exist, skipping NOTICE: table "pgbench_tellers" does not exist, skipping NOTICE: table "pgbench_accounts" does not exist, skipping NOTICE: table "pgbench_branches" does not exist, skipping creating tables... 100000 of 2000000 tuples (5%) done (elapsed 0.11 s, remaining 2.10 s) 200000 of 2000000 tuples (10%) done (elapsed 0.22 s, remaining 1.98 s) 300000 of 2000000 tuples (15%) done (elapsed 0.33 s, remaining 1.84 s) 400000 of 2000000 tuples (20%) done (elapsed 0.42 s, remaining 1.67 s) 500000 of 2000000 tuples (25%) done (elapsed 0.52 s, remaining 1.57 s) 600000 of 2000000 tuples (30%) done (elapsed 0.62 s, remaining 1.45 s) 700000 of 2000000 tuples (35%) done (elapsed 0.73 s, remaining 1.35 s) 800000 of 2000000 tuples (40%) done (elapsed 0.87 s, remaining 1.31 s) 900000 of 2000000 tuples (45%) done (elapsed 0.98 s, remaining 1.19 s) 1000000 of 2000000 tuples (50%) done (elapsed 1.09 s, remaining 1.09 s) 1100000 of 2000000 tuples (55%) done (elapsed 1.22 s, remaining 1.00 s) 1200000 of 2000000 tuples (60%) done (elapsed 1.36 s, remaining 0.91 s) 1300000 of 2000000 tuples (65%) done (elapsed 1.51 s, remaining 0.82 s) 1400000 of 2000000 tuples (70%) done (elapsed 1.65 s, remaining 0.71 s) 1500000 of 2000000 tuples (75%) done (elapsed 1.78 s, remaining 0.59 s) 1600000 of 2000000 tuples (80%) done (elapsed 1.93 s, remaining 0.48 s) 1700000 of 2000000 tuples (85%) done (elapsed 2.10 s, remaining 0.37 s) 1800000 of 2000000 tuples (90%) done (elapsed 2.23 s, remaining 0.25 s) 1900000 of 2000000 tuples (95%) done (elapsed 2.37 s, remaining 0.12 s) 2000000 of 2000000 tuples (100%) done (elapsed 2.48 s, remaining 0.00 s) vacuum... set primary keys... done.
A total of 500,00 transactions are then processed. This will populate the pgbench_history table with 500,000 rows.
pgbench -U enterprisedb -t 500000 bank starting vacuum...end. transaction type: <builtin: TPC-B (sort of)> scaling factor: 20 query mode: simple number of clients: 1 number of threads: 1 number of transactions per client: 500000 number of transactions actually processed: 500000/500000 latency average: 0.000 ms tps = 1464.338375 (including connections establishing) tps = 1464.350357 (excluding connections establishing)
The table definitions are shown below:
\d pgbench_accounts
Table "public.pgbench_accounts"
Column | Type | Modifiers
----------+---------------+-----------
aid | integer | not null
bid | integer |
abalance | integer |
filler | character(84) |
Indexes:
"pgbench_accounts_pkey" PRIMARY KEY, btree (aid)
\d pgbench_branches
Table "public.pgbench_branches"
Column | Type | Modifiers
----------+---------------+-----------
bid | integer | not null
bbalance | integer |
filler | character(88) |
Indexes:
"pgbench_branches_pkey" PRIMARY KEY, btree (bid)
\d pgbench_tellers
Table "public.pgbench_tellers"
Column | Type | Modifiers
----------+---------------+-----------
tid | integer | not null
bid | integer |
tbalance | integer |
filler | character(84) |
Indexes:
"pgbench_tellers_pkey" PRIMARY KEY, btree (tid)
\d pgbench_history
Table "public.pgbench_history"
Column | Type | Modifiers
--------+-----------------------------+-----------
tid | integer |
bid | integer |
aid | integer |
delta | integer |
mtime | timestamp without time zone |
filler | character(22) |The EXPLAIN command shows the plan selected by the query planner. In the following example, aid is the primary key column, so an indexed search is used on index, pgbench_accounts_pkey.
EXPLAIN SELECT * FROM pgbench_accounts WHERE aid = 100;
QUERY PLAN
-----------------------------------------------------------------------------
------------------
Index Scan using pgbench_accounts_pkey on pgbench_accounts (cost=0.43..8.45
rows=1 width=97)
Index Cond: (aid = 100)
(2 rows)The FULL hint is used to force a full sequential scan instead of using the index as shown below:
EXPLAIN SELECT /*+ FULL(pgbench_accounts) */ * FROM pgbench_accounts WHERE
aid = 100;
QUERY PLAN
---------------------------------------------------------------------
Seq Scan on pgbench_accounts (cost=0.00..58781.69 rows=1 width=97)
Filter: (aid = 100)
(2 rows)The NO_INDEX hint forces a parallel sequential scan instead of use of the index as shown below:
EXPLAIN SELECT /*+ NO_INDEX(pgbench_accounts pgbench_accounts_pkey) */ *
FROM pgbench_accounts WHERE aid = 100;
QUERY PLAN
-----------------------------------------------------------------------------
-------
Gather (cost=1000.00..45094.80 rows=1 width=97)
Workers Planned: 2
-> Parallel Seq Scan on pgbench_accounts (cost=0.00..44094.70 rows=1
width=97)
Filter: (aid = 100)
(4 rows)In addition to using the EXPLAIN command as shown in the prior examples, more detailed information regarding whether or not a hint was used by the planner can be obtained by setting the trace_hints configuration parameter as follows:
SET trace_hints TO on;
The SELECT command with the NO_INDEX hint is repeated below to illustrate the additional information produced when the trace_hints configuration parameters is set.
EXPLAIN SELECT /*+ NO_INDEX(pgbench_accounts pgbench_accounts_pkey) */ *
FROM pgbench_accounts WHERE aid = 100;
INFO: [HINTS] Index Scan of [pgbench_accounts].[pgbench_accounts_pkey]
rejected due to NO_INDEX hint.
QUERY PLAN
-----------------------------------------------------------------------------
-------
Gather (cost=1000.00..45094.80 rows=1 width=97)
Workers Planned: 2
-> Parallel Seq Scan on pgbench_accounts (cost=0.00..44094.70 rows=1
width=97)
Filter: (aid = 100)
(4 rows)Note that if a hint is ignored, the INFO: [HINTS] line will not appear. This may be an indication that there was a syntax error or some other misspelling in the hint as shown in the following example where the index name is misspelled.
EXPLAIN SELECT /*+ NO_INDEX(pgbench_accounts pgbench_accounts_xxx) */ * FROM
pgbench_accounts WHERE aid = 100;
QUERY PLAN
-----------------------------------------------------------------------------
------------------
Index Scan using pgbench_accounts_pkey on pgbench_accounts (cost=0.43..8.45
rows=1 width=97)
Index Cond: (aid = 100)
(2 rows)