There is no syntax variant that lets you update the whole row at once. However, there is a shorter form than what you have so far.
Also, you do not actually want to update all columns. The WHERE
condition on id pins down at least one column (id
) to remain unchanged. But that's just nitpicking.
UPDATE table_a a
SET ( c1, c2, ...)
= (b.c1, b.c2, ...)
FROM table_b b
WHERE a.id = b.id;
More details in this related answer:
Bulk update of all columns
DELETE / INSERT
Internally, due to the MVCC model of Postgres, every UPDATE
effectively inserts a new row anyway and marks the old one as obsolete. So, behind the curtains there is not much difference between UPDATE
and DELETE
plus INSERT
.
There are some details in favor of the UPDATE
route:
- HOT UPDATE.
- TOAST tables: If you have large columns, the content may be stored."out-of-line" in TOAST tables and the new row version can link to the same row in the TOAST table if toasted columns remain unchanged.
- Index maintenance may be cheaper for updates.
Otherwise, locking should be about the same. You need an exclusive lock on affected rows either way. Just make it quick.
If you are dealing with a huge number of rows and you don't need a consistent state (all rows or none), you can split the operation into multiple batches. (Separate transactions!) Increases the total cost, but keeps the lock time per row shorter.
1st case
You seem to forget the valid_during
range. As your third case suggests, there can be multiple entries per (rec_id, val)
, so you must select the right one:
UPDATE master m
SET valid_on = f_array_sort(m.valid_on || u.valid_on) -- sorted array, see below
FROM updates u
WHERE m.rec_id = u.rec_id
AND m.valid_during @> u.valid_on -- additional check
AND m.val = u.val
AND NOT m.valid_on @> ARRAY[u.valid_on];
I assume the whole possible date range is always covered for each existing rec_id
and valid_during
shall not overlap per rec_id
, or you'd have to do more.
After installing the additional module btree_gist
, add an exclusion constraint to rule out overlapping date ranges if you don't have one, yet:
ALTER TABLE master ADD CONSTRAINT EXCLUDE
USING gist (rec_id WITH =, valid_during WITH &&) -- disallow overlap
The GiST index this is implemented with is also a perfect match for the query. Details:
2nd / 3rd case
Assuming that every date range starts with the smallest date in the (now sorted!) array: lower(m.valid_during) = m.valid_on[1]
. I would enforce that with a CHECK
constraint.
Here we need to create one or two new rows
In the 2nd case it is enough to shrink the range of the old row and insert one new row
In the 3rd case we update the old row with the left half of array and range, insert the new row and finally insert the with the right half of array and range.
Helper functions
To keep it simple I introduce a new constraint: every array is sorted. Use this helper function
-- sort array
CREATE OR REPLACE FUNCTION f_array_sort(anyarray)
RETURNS anyarray LANGUAGE sql IMMUTABLE AS
$$SELECT ARRAY (SELECT unnest($1) ORDER BY 1)$$;
I don't need your helper function arraymin()
any more, but it could be simplified to:
CREATE OR REPLACE FUNCTION f_array_min(anyarray)
RETURNS anyelement LANGUAGE sql IMMUTABLE AS
$$SELECT min(a) FROM unnest($1) a$$;
Two more to get the left and right half of an array split at a given element:
-- split left array at given element
CREATE OR REPLACE FUNCTION f_array_left(anyarray, anyelement)
RETURNS anyarray LANGUAGE sql IMMUTABLE AS
$$SELECT ARRAY (SELECT * FROM unnest($1) a WHERE a < $2 ORDER BY 1)$$;
-- split right array at given element
CREATE OR REPLACE FUNCTION f_array_right(anyarray, anyelement)
RETURNS anyarray LANGUAGE sql IMMUTABLE AS
$$SELECT ARRAY (SELECT * FROM unnest($1) a WHERE a >= $2 ORDER BY 1)$$;
Query
This does all the rest:
WITH u AS ( -- identify candidates
SELECT m.id, rec_id, m.val, m.valid_on, m.valid_during
, u.val AS u_val, u.valid_on AS u_valid_on
FROM master m
JOIN updates u USING (rec_id)
WHERE m.val <> u.val
AND m.valid_during @> u.valid_on
FOR UPDATE -- lock for update
)
, upd1 AS ( -- case 2: no overlap, no split
UPDATE master m -- shrink old row
SET valid_during = daterange(lower(u.valid_during), u.u_valid_on)
FROM u
WHERE u.id = m.id
AND u.u_valid_on > m.valid_on[array_upper(m.valid_on, 1)]
RETURNING m.id
)
, ins1 AS ( -- insert new row
INSERT INTO master (rec_id, val, valid_on, valid_during)
SELECT u.rec_id, u.u_val, ARRAY[u.u_valid_on]
, daterange(u.u_valid_on, upper(u.valid_during))
FROM upd1
JOIN u USING (id)
)
, upd2 AS ( -- case 3: overlap, need to split row
UPDATE master m -- shrink to first half
SET valid_during = daterange(lower(u.valid_during), u.u_valid_on)
, valid_on = f_array_left(u.valid_on, u.u_valid_on)
FROM u
LEFT JOIN upd1 USING (id)
WHERE upd1.id IS NULL -- all others
AND u.id = m.id
RETURNING m.id, f_array_right(u.valid_on, u.u_valid_on) AS arr_right
)
INSERT INTO master (rec_id, val, valid_on, valid_during)
-- new row
SELECT u.rec_id, u.u_val, ARRAY[u.u_valid_on]
, daterange(u.u_valid_on, upd2.arr_right[1])
FROM upd2
JOIN u USING (id)
UNION ALL -- second half of old row
SELECT u.rec_id, u.val, upd2.arr_right
, daterange(upd2.arr_right[1], upper(u.valid_during))
FROM upd2
JOIN u USING (id);
SQL Fiddle.
Notes
You need to understand the concept of data-modifying CTEs (writeable CTEs), before you touch this. Judging from the code you provided, you know your way around Postgres.
FOR UPDATE
is to avoid race conditions with concurrent write access. If you are the only user writing to the tables, you don't need it.
I took a piece of paper and drew a timeline so not to get lost in all of this.
Each row is only updated / inserted once, and operations are simple and roughly optimized. No expensive window functions. This should perform well. Much faster than your previous approach in any case.
It would be a bit less confusing if you'd use distinct column names for u.valid_on
and m.valid_on
, which are related but different things.
I compute the right half of the split array in the RETURNING
clause of CTE upd2
: f_array_right(u.valid_on, u.u_valid_on) AS arr_right
, because I need it several times in the next step. This is a (legal) trick to save one more CTE.
As for solutions that don't involve unnesting the master table
: You have to unnest the array valid_on
either way, to split it, at least as long as it's not sorted. Also, your helper function arraymin()
is already unnesting it anyway.
Best Answer
You can use a data modifying CTE to do this in a single statement:
from changed_user