In case the query optimizer ignores the index, you can use the FORCE INDEX hint to instruct it to use the index instead. In this syntax, you put the FORCE INDEX clause after the FROM clause followed by a list of named indexes that the query optimizer must use.
As we saw above, running a couple of queries on our posts table reveals that even given an index to use, Postgres will not always choose to use it. The reason why this is the case is that indexes have a cost to create and maintain (on writes) and use (on reads).
If the SELECT returns more than approximately 5-10% of all rows in the table, a sequential scan is much faster than an index scan. This is because an index scan requires several IO operations for each row (look up the row in the index, then retrieve the row from the heap).
PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. The index covers the columns that make up the primary key or unique constraint (a multicolumn index, if appropriate), and is the mechanism that enforces the constraint.
Assuming you're asking about the common "index hinting" feature found in many databases, PostgreSQL doesn't provide such a feature. This was a conscious decision made by the PostgreSQL team. A good overview of why and what you can do instead can be found here. The reasons are basically that it's a performance hack that tends to cause more problems later down the line as your data changes, whereas PostgreSQL's optimizer can re-evaluate the plan based on the statistics. In other words, what might be a good query plan today probably won't be a good query plan for all time, and index hints force a particular query plan for all time.
As a very blunt hammer, useful for testing, you can use the enable_seqscan
and enable_indexscan
parameters. See:
enable_
parametersThese are not suitable for ongoing production use. If you have issues with query plan choice, you should see the documentation for tracking down query performance issues. Don't just set enable_
params and walk away.
Unless you have a very good reason for using the index, Postgres may be making the correct choice. Why?
See also this old newsgroup post.
Probably the only valid reason for using
set enable_seqscan=false
is when you're writing queries and want to quickly see what the query plan would actually be were there large amounts of data in the table(s). Or of course if you need to quickly confirm that your query is not using an index simply because the dataset is too small.
This problem typically happens when the query planner's estimated cost of an index scan is too high and doesn't correctly reflect reality. To fix this, you need to lower the random_page_cost
configuration parameter. From the Postgres documentation:
Reducing this value [...] will cause the system to prefer index scans; raising it will make index scans look relatively more expensive.
You can do a quick test whether this will actually make Postgres use the index:
EXPLAIN <query>; # Uses sequential scan
SET random_page_cost = 1;
EXPLAIN <query>; # May use index scan now
You can restore the default value with SET random_page_cost = DEFAULT;
again. You can change the global default permanently with ALTER SYSTEM SET random_page_cost = 1;
Index scans require non-sequential disk page fetches. Postgres uses random_page_cost
to estimate the cost of such non-sequential fetches in relation to sequential fetches. The default value is 4.0
, thus assuming an average cost factor of 4 compared to sequential fetches (taking caching effects into account).
The problem however is that this default value is unsuitable in the following scenarios:
1) Cached indices
If an index is already cached in RAM, then an index scan will always be significantly faster than a sequential scan. The query planner however doesn't exactly know which parts of the index are already cached, and thus might make an incorrect decision. The Postgres documentation says:
If your data is likely to be completely in cache, [...] decreasing
random_page_cost
can be appropriate.
So, how do you know whether "your data is likely to be cached"? Well, if a specific index is frequently used, and if the system has sufficient RAM, then data is likely to be cached eventually, and random_page_cost
should be set to a lower value. You'll have to experiment with different values and see what works for you.
You could also use the pg_prewarm extension for explicit data caching.
2) Solid-state drives
As per the documentation:
Storage that has a low random read cost relative to sequential, e.g., solid-state drives, might also be better modeled with a lower value for
random_page_cost
, e.g.,1.1
.
This slide from a speak at PostgresConf 2018 also says that random_page_cost
should be set to something between 1.0
and 2.0
for solid-state drives.
Sometimes PostgreSQL fails to make the best choice of indexes for a particular condition. As an example, suppose there is a transactions table with several million rows, of which there are several hundred for any given day, and the table has four indexes: transaction_id, client_id, date, and description. You want to run the following query:
SELECT client_id, SUM(amount)
FROM transactions
WHERE date >= 'yesterday'::timestamp AND date < 'today'::timestamp AND
description = 'Refund'
GROUP BY client_id
PostgreSQL may choose to use the index transactions_description_idx instead of transactions_date_idx, which may lead to the query taking several minutes instead of less than one second. If this is the case, you can force using the index on date by fudging the condition like this:
SELECT client_id, SUM(amount)
FROM transactions
WHERE date >= 'yesterday'::timestamp AND date < 'today'::timestamp AND
description||'' = 'Refund'
GROUP BY client_id
The question on itself is very much invalid. Forcing (by doing enable_seqscan=off for example) is very bad idea. It might be useful to check if it will be faster, but production code should never use such tricks.
Instead - do explain analyze of your query, read it, and find out why PostgreSQL chooses bad (in your opinion) plan.
There are tools on the web that help with reading explain analyze output - one of them is explain.depesz.com - written by me.
Another option is to join #postgresql channel on freenode irc network, and talking to guys there to help you out - as optimizing query is not a matter of "ask a question, get answer be happy". it's more like a conversation, with many things to check, many things to be learned.
One thing to note with PostgreSQL; where you are expecting an index to be used and it is not being used, is to VACUUM ANALYZE the table.
VACUUM ANALYZE schema.table;
This updates statistics used by the planner to determine the most efficient way to execute a query. Which may result in the index being used.
There is a trick to push postgres to prefer a seqscan adding a OFFSET 0
in the subquery
This is handy for optimizing requests linking big/huge tables when all you need is only the n first/last elements.
Lets say you are looking for first/last 20 elements involving multiple tables having 100k (or more) entries, no point building/linking up all the query over all the data when what you'll be looking for is in the first 100 or 1000 entries. In this scenario for example, it turns out to be over 10x faster to do a sequential scan.
see How can I prevent Postgres from inlining a subquery?
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With