Postgresql query containing a constant in a logical-or leads to full table scan
I have a query that contains a clause of the form CONSTANT OR t.foo=:foo, where CONSTANT is some expression that evaluates to a constant boolean value. Then the query is executed as a full table scan, even when there is an index on column foo and the simple t.foo=:foo would use the index. The example query that I analyzed had the form :foo IS NOT NULL OR t.foo=:foo. Here is a simple example:
CREATE TABLE tst (
id serial,
name VARCHAR NOT NULL,
age INTEGER NOT NULL,
CONSTRAINT PK_ID PRIMARY KEY(ID));
EXPLAIN SELECT * FROM tst WHERE (23 IS NOT NULL OR id=23);
QUERY PLAN
--------------------------------------------------------
Seq Scan on tst (cost=0.00..22.00 rows=1200 width=40)
Is there any reason for this? Is there a way to tell the optimizer to logically evaluate/simplify the query?
postgresql query-optimization boolean-logic
asked Nov 12 '18 at 19:09
EPSG31468EPSG31468
688416
add a comment |
I have a query that contains a clause of the form CONSTANT OR t.foo=:foo, where CONSTANT is some expression that evaluates to a constant boolean value. Then the query is executed as a full table scan, even when there is an index on column foo and the simple t.foo=:foo would use the index. The example query that I analyzed had the form :foo IS NOT NULL OR t.foo=:foo. Here is a simple example:
CREATE TABLE tst (
id serial,
name VARCHAR NOT NULL,
age INTEGER NOT NULL,
CONSTRAINT PK_ID PRIMARY KEY(ID));
EXPLAIN SELECT * FROM tst WHERE (23 IS NOT NULL OR id=23);
QUERY PLAN
--------------------------------------------------------
Seq Scan on tst (cost=0.00..22.00 rows=1200 width=40)
Is there any reason for this? Is there a way to tell the optimizer to logically evaluate/simplify the query?
postgresql query-optimization boolean-logic
asked Nov 12 '18 at 19:09
EPSG31468EPSG31468
688416
3
where (23 IS NOT NULL OR id=23)is the same aswhere (true or id = 23)which is the same aswhere truewhich is the same as not having awherecondition at all. So you request all rows of the table, and the most efficient way to do that is a Seq Scan
– a_horse_with_no_name
Nov 12 '18 at 19:38
You are right, this was totally non-sense. I tried to simplify a more complex example, but this is of cause a full-table scan because the WHERE-part is true.
– EPSG31468
Nov 17 '18 at 18:44
add a comment |
I have a query that contains a clause of the form CONSTANT OR t.foo=:foo, where CONSTANT is some expression that evaluates to a constant boolean value. Then the query is executed as a full table scan, even when there is an index on column foo and the simple t.foo=:foo would use the index. The example query that I analyzed had the form :foo IS NOT NULL OR t.foo=:foo. Here is a simple example:
CREATE TABLE tst (
id serial,
name VARCHAR NOT NULL,
age INTEGER NOT NULL,
CONSTRAINT PK_ID PRIMARY KEY(ID));
EXPLAIN SELECT * FROM tst WHERE (23 IS NOT NULL OR id=23);
QUERY PLAN
--------------------------------------------------------
Seq Scan on tst (cost=0.00..22.00 rows=1200 width=40)
Is there any reason for this? Is there a way to tell the optimizer to logically evaluate/simplify the query?
postgresql query-optimization boolean-logic
asked Nov 12 '18 at 19:09
EPSG31468EPSG31468
688416
I have a query that contains a clause of the form CONSTANT OR t.foo=:foo, where CONSTANT is some expression that evaluates to a constant boolean value. Then the query is executed as a full table scan, even when there is an index on column foo and the simple t.foo=:foo would use the index. The example query that I analyzed had the form :foo IS NOT NULL OR t.foo=:foo. Here is a simple example:
CREATE TABLE tst (
id serial,
name VARCHAR NOT NULL,
age INTEGER NOT NULL,
CONSTRAINT PK_ID PRIMARY KEY(ID));
EXPLAIN SELECT * FROM tst WHERE (23 IS NOT NULL OR id=23);
QUERY PLAN
--------------------------------------------------------
Seq Scan on tst (cost=0.00..22.00 rows=1200 width=40)
Is there any reason for this? Is there a way to tell the optimizer to logically evaluate/simplify the query?
postgresql query-optimization boolean-logic
postgresql query-optimization boolean-logic
asked Nov 12 '18 at 19:09
EPSG31468EPSG31468
688416
asked Nov 12 '18 at 19:09
EPSG31468EPSG31468
688416
asked Nov 12 '18 at 19:09
EPSG31468EPSG31468
688416
asked Nov 12 '18 at 19:09
EPSG31468EPSG31468
688416
asked Nov 12 '18 at 19:09
EPSG31468EPSG31468
688416
688416
3
where (23 IS NOT NULL OR id=23)is the same aswhere (true or id = 23)which is the same aswhere truewhich is the same as not having awherecondition at all. So you request all rows of the table, and the most efficient way to do that is a Seq Scan
– a_horse_with_no_name
Nov 12 '18 at 19:38
You are right, this was totally non-sense. I tried to simplify a more complex example, but this is of cause a full-table scan because the WHERE-part is true.
– EPSG31468
Nov 17 '18 at 18:44
add a comment |
3
where (23 IS NOT NULL OR id=23)is the same aswhere (true or id = 23)which is the same aswhere truewhich is the same as not having awherecondition at all. So you request all rows of the table, and the most efficient way to do that is a Seq Scan
– a_horse_with_no_name
Nov 12 '18 at 19:38
You are right, this was totally non-sense. I tried to simplify a more complex example, but this is of cause a full-table scan because the WHERE-part is true.
– EPSG31468
Nov 17 '18 at 18:44
3
3
where (23 IS NOT NULL OR id=23) is the same as where (true or id = 23) which is the same as where true which is the same as not having a where condition at all. So you request all rows of the table, and the most efficient way to do that is a Seq Scan– a_horse_with_no_name
Nov 12 '18 at 19:38
where (23 IS NOT NULL OR id=23) is the same as where (true or id = 23) which is the same as where true which is the same as not having a where condition at all. So you request all rows of the table, and the most efficient way to do that is a Seq Scan– a_horse_with_no_name
Nov 12 '18 at 19:38
You are right, this was totally non-sense. I tried to simplify a more complex example, but this is of cause a full-table scan because the WHERE-part is true.
– EPSG31468
Nov 17 '18 at 18:44
You are right, this was totally non-sense. I tried to simplify a more complex example, but this is of cause a full-table scan because the WHERE-part is true.
– EPSG31468
Nov 17 '18 at 18:44
add a comment |
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where (23 IS NOT NULL OR id=23)is the same aswhere (true or id = 23)which is the same aswhere truewhich is the same as not having awherecondition at all. So you request all rows of the table, and the most efficient way to do that is a Seq Scan– a_horse_with_no_name
Nov 12 '18 at 19:38
You are right, this was totally non-sense. I tried to simplify a more complex example, but this is of cause a full-table scan because the WHERE-part is true.
– EPSG31468
Nov 17 '18 at 18:44