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Diffstat (limited to 'doc/FAQ')
| -rw-r--r-- | doc/FAQ | 15 |
1 files changed, 9 insertions, 6 deletions
@@ -1,7 +1,7 @@ Frequently Asked Questions (FAQ) for PostgreSQL - Last updated: Mon Sep 30 23:28:35 EDT 2002 + Last updated: Wed Oct 9 23:14:53 EDT 2002 Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) @@ -998,18 +998,21 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP ); 4.22) Why are my subqueries using IN so slow? Currently, we join subqueries to outer queries by sequentially - scanning the result of the subquery for each row of the outer query. A - workaround is to replace IN with EXISTS: + scanning the result of the subquery for each row of the outer query. + If the subquery returns only a few rows and the outer query returns + many rows, IN is fastest. To speed up other queries, replace IN with + EXISTS: SELECT * FROM tab - WHERE col1 IN (SELECT col2 FROM TAB2) + WHERE col IN (SELECT subcol FROM subtab) to: SELECT * FROM tab - WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2) + WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col) - We hope to fix this limitation in a future release. + For this to be fast, subcol should be an indexed column. We hope to + fix this limitation in a future release. 4.23) How do I perform an outer join? |