to_char in where clause SQL / Postgresql

Question

I'm using Postgresql DB.

I want to perform a query against a numeric column in a table to find if the value starts with a particular number.

I currently have:

SELECT * FROM myTable WHERE to_char(ID, '12345678') LIKE '2%'

However this returns an empty dataset (but there are values in the ID column which start with 2.

Many thanks

Steve

Answer 1

Don't laugh. As an alternative to the casts, you could use a range query, like:

SELECT * FROM mytable
WHERE id = 2
OR (id >=20 AND id < 30)
OR (id >=200 AND id < 300)
OR (id >=2000 AND id < 3000)
OR (id >=20000 AND id < 30000)
OR (id >=200000 AND id < 300000)
OR (id >=2000000 AND id < 3000000)
OR (id >=20000000 AND id < 30000000)
OR (id >=200000000 AND id < 300000000)
OR (id >=2000000000 AND id < 3000000000)
OR (id >=20000000000 AND id < 30000000000)
OR (id >=200000000000 AND id < 300000000000)
        -- ...
        ;

This could cause postgres to generate a much better query plan, because an index can be used (if present, which can be expected for an id field)

UPDATE: The query plans:

Bitmap Heap Scan on reservations  (cost=1838.15..13290.11 rows=110809 width=4012) (actual time=11.310..24.379 rows=111111 loops=1)
   Recheck Cond: ((id = 2) OR ((id >= 20) AND (id < 30)) OR ((id >= 200) AND (id < 300)) OR ((id >= 2000) AND (id < 3000)) OR ((id >= 20000) AND (id < 30000)) OR ((id >= 200000) AND (id < 300000)) OR ((id >= 2000000) AND (id < 3000000)) OR ((id >= 20000000) AND (id < 30000000)) OR ((id >= 200000000) AND (id < 300000000)) OR ((id >= 2000000000) AND (id < 3000000000::bigint)) OR ((id >= 20000000000::bigint) AND (id < 30000000000::bigint)) OR ((id >= 200000000000::bigint) AND (id < 300000000000::bigint)))
   ->  BitmapOr  (cost=1838.15..1838.15 rows=112192 width=0) (actual time=11.242..11.242 rows=0 loops=1)
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..1.49 rows=1 width=0) (actual time=0.005..0.005 rows=1 loops=1)
               Index Cond: (id = 2)
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..1.58 rows=10 width=0) (actual time=0.003..0.003 rows=10 loops=1)
               Index Cond: ((id >= 20) AND (id < 30))
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..2.52 rows=104 width=0) (actual time=0.013..0.013 rows=100 loops=1)
               Index Cond: ((id >= 200) AND (id < 300))
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..14.24 rows=1036 width=0) (actual time=0.110..0.110 rows=1000 loops=1)
               Index Cond: ((id >= 2000) AND (id < 3000))
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..144.73 rows=10845 width=0) (actual time=1.050..1.050 rows=10000 loops=1)
               Index Cond: ((id >= 20000) AND (id < 30000))
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..1332.24 rows=100196 width=0) (actual time=10.013..10.013 rows=100000 loops=1)
               Index Cond: ((id >= 200000) AND (id < 300000))
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..1.49 rows=1 width=0) (actual time=0.010..0.010 rows=0 loops=1)
               Index Cond: ((id >= 2000000) AND (id < 3000000))
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..1.49 rows=1 width=0) (actual time=0.001..0.001 rows=0 loops=1)
               Index Cond: ((id >= 20000000) AND (id < 30000000))
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..1.49 rows=1 width=0) (actual time=0.001..0.001 rows=0 loops=1)
               Index Cond: ((id >= 200000000) AND (id < 300000000))
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..1.49 rows=1 width=0) (actual time=0.002..0.002 rows=0 loops=1)
               Index Cond: ((id >= 2000000000) AND (id < 3000000000::bigint))
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..1.49 rows=1 width=0) (actual time=0.026..0.026 rows=0 loops=1)
               Index Cond: ((id >= 20000000000::bigint) AND (id < 30000000000::bigint))
         ->  Bitmap Index Scan on reservations_pkey  (cost=0.00..1.49 rows=1 width=0) (actual time=0.002..0.002 rows=0 loops=1)
               Index Cond: ((id >= 200000000000::bigint) AND (id < 300000000000::bigint))
 Total runtime: 28.720 ms
(28 rows)


 Seq Scan on reservations  (cost=0.00..19219.52 rows=4383 width=4012) (actual time=0.025..184.532 rows=111111 loops=1)
   Filter: ((id)::text ~~ '2%'::text)
 Total runtime: 189.100 ms
(3 rows)

Answer 2

That's because the pattern for a number in TO_CHAR is either 9 (value with the specified number of digits) or 0 (value with leading zeros). Either use the following:

SELECT * FROM myTable WHERE TRIM(to_char(ID, '99999999')) LIKE '2%'

Or cast the number to a char.

Answer 3

This will return the leading digit:

floor(a/10.^(floor(log(a))))

Example:

select a from generate_series(1,1000) as a where 
    floor(a/10.^(floor(log(a)))) = 2;

If you have numbers which are smaller than one or less than zero you should filter them out (depending on what you want exactly).

Answer 4

I found the answer:

SELECT * FROM myTable WHERE CAST (ID AS CHAR) LIKE '2%'

Steve

Answer 5

You dont need the to_char, just id like '2%'.