On Jul 7, 2008, at 08:01, Andrew Dunstan wrote:
> What does still bother me is its performance. I'd like to know if
> any measurement has been done of using citext vs. a functional index
> on lower(foo).
Okay, here's a start. The attached script inserts random strings of
1-10 space-delimited words into text and citext columns, and then
compares the performance of queries with and without indexes. The
output for me is as follows:
Loading words from dictionary.
Inserting into the table.
Test =.
SELECT * FROM try WHERE LOWER(text) = LOWER('food');
Time: 254.254 ms
SELECT * FROM try WHERE citext = 'food';
Time: 288.535 ms
Test LIKE and ILIKE
SELECT * FROM try WHERE LOWER(text) LIKE LOWER('C%');
Time: 209.385 ms
SELECT * FROM try WHERE citext ILIKE 'C%';
Time: 236.186 ms
SELECT * FROM try WHERE citext LIKE 'C%';
Time: 235.818 ms
Adding indexes...
Test =.
SELECT * FROM try WHERE LOWER(text) = LOWER('food');
Time: 1.260 ms
SELECT * FROM try WHERE citext = 'food';
Time: 277.755 ms
Test LIKE and ILIKE
SELECT * FROM try WHERE LOWER(text) LIKE LOWER('C%');
Time: 209.073 ms
SELECT * FROM try WHERE citext ILIKE 'C%';
Time: 238.430 ms
SELECT * FROM try WHERE citext LIKE 'C%';
Time: 238.685 ms
benedict%
So for some reason, after adding the indexes, the queries against the
CITEXT column aren't using them. Furthermore, the `lower(text) LIKE
lower(?)` query isn't using *its* index. Huh?
So this leaves me with two questions:
1. For what reason would the query against the citext column *not* use
the index?
2. Is there some way to get the CITEXT index to behave like a LOWER()
index, that is, so that its value is stored using the result of the
str_tolower() function, thus removing some of the overhead of
converting the values for each row fetched from the index? (Does this
question make any sense?)
Thanks,
David
