14.3. Controlling the Planner with Explicit
It is possible
to control the query planner to some extent by using the explicit
syntax. To see why this matters, we first need some background.
In a simple join query, such as:
SELECT * FROM a, b, c WHERE a.id = b.id AND b.ref = c.id;
the planner is free to join the given tables in any order. For
example, it could generate a query plan that joins A to B, using
a.id = b.id, and then
joins C to this joined table, using the other
condition. Or it could join B to C and then join A to that result.
Or it could join A to C and then join them with B — but that
would be inefficient, since the full Cartesian product of A and C
would have to be formed, there being no applicable condition in the
WHERE clause to allow optimization of the join. (All
joins in the PostgreSQL executor happen
between two input tables, so it's necessary to build up the result
in one or another of these fashions.) The important point is that
these different join possibilities give semantically equivalent
results but might have hugely different execution costs. Therefore,
the planner will explore all of them to try to find the most
efficient query plan.
When a query only involves two or three tables, there aren't many join orders to worry about. But the number of possible join orders grows exponentially as the number of tables expands. Beyond ten or so input tables it's no longer practical to do an exhaustive search of all the possibilities, and even for six or seven tables planning might take an annoyingly long time. When there are too many input tables, the PostgreSQL planner will switch from exhaustive search to a genetic probabilistic search through a limited number of possibilities. (The switch-over threshold is set by the geqo_threshold run-time parameter.) The genetic search takes less time, but it won't necessarily find the best possible plan.
When the query involves outer joins, the planner has less freedom than it does for plain (inner) joins. For example, consider:
SELECT * FROM a LEFT JOIN (b JOIN c ON (b.ref = c.id)) ON (a.id = b.id);
Although this query's restrictions are superficially similar to the previous example, the semantics are different because a row must be emitted for each row of A that has no matching row in the join of B and C. Therefore the planner has no choice of join order here: it must join B to C and then join A to that result. Accordingly, this query takes less time to plan than the previous query. In other cases, the planner might be able to determine that more than one join order is safe. For example, given:
SELECT * FROM a LEFT JOIN b ON (a.bid = b.id) LEFT JOIN c ON (a.cid = c.id);
it is valid to join A to either B or C first. Currently, only
FULL JOIN completely constrains the join order. Most
practical cases involving
LEFT JOIN or
can be rearranged to some extent.
Explicit inner join syntax (
JOIN, or unadorned
JOIN) is semantically the same as
listing the input relations in
FROM, so it does not
constrain the join order.
Even though most kinds of
JOIN don't completely constrain
the join order, it is possible to instruct the
PostgreSQL query planner to treat all
JOIN clauses as constraining the join order anyway.
For example, these three queries are logically equivalent:
SELECT * FROM a, b, c WHERE a.id = b.id AND b.ref = c.id; SELECT * FROM a CROSS JOIN b CROSS JOIN c WHERE a.id = b.id AND b.ref = c.id; SELECT * FROM a JOIN (b JOIN c ON (b.ref = c.id)) ON (a.id = b.id);
But if we tell the planner to honor the
the second and third take less time to plan than the first. This effect
is not worth worrying about for only three tables, but it can be a
lifesaver with many tables.
To force the planner to follow the join order laid out by explicit
set the join_collapse_limit run-time parameter to 1.
(Other possible values are discussed below.)
You do not need to constrain the join order completely in order to
cut search time, because it's OK to use
within items of a plain
FROM list. For example, consider:
SELECT * FROM a CROSS JOIN b, c, d, e WHERE ...;
join_collapse_limit = 1, this
forces the planner to join A to B before joining them to other tables,
but doesn't constrain its choices otherwise. In this example, the
number of possible join orders is reduced by a factor of 5.
Constraining the planner's search in this way is a useful technique
both for reducing planning time and for directing the planner to a
good query plan. If the planner chooses a bad join order by default,
you can force it to choose a better order via
— assuming that you know of a better order, that is. Experimentation
A closely related issue that affects planning time is collapsing of subqueries into their parent query. For example, consider:
SELECT * FROM x, y, (SELECT * FROM a, b, c WHERE something) AS ss WHERE somethingelse;
This situation might arise from use of a view that contains a join;
SELECT rule will be inserted in place of the view
reference, yielding a query much like the above. Normally, the planner
will try to collapse the subquery into the parent, yielding:
SELECT * FROM x, y, a, b, c WHERE something AND somethingelse;
This usually results in a better plan than planning the subquery
separately. (For example, the outer
WHERE conditions might be such that
joining X to A first eliminates many rows of A, thus avoiding the need to
form the full logical output of the subquery.) But at the same time,
we have increased the planning time; here, we have a five-way join
problem replacing two separate three-way join problems. Because of the
exponential growth of the number of possibilities, this makes a big
difference. The planner tries to avoid getting stuck in huge join search
problems by not collapsing a subquery if more than
FROM items would result in the parent
query. You can trade off planning time against quality of plan by
adjusting this run-time parameter up or down.
from_collapse_limit and join_collapse_limit
are similarly named because they do almost the same thing: one controls
when the planner will “flatten out” subqueries, and the
other controls when it will flatten out explicit joins. Typically
you would either set
join_collapse_limit equal to
from_collapse_limit (so that explicit joins and subqueries
act similarly) or set
join_collapse_limit to 1 (if you want
to control join order with explicit joins). But you might set them
differently if you are trying to fine-tune the trade-off between planning
time and run time.