Update IN/EXISTS item.

1 files changed, 9 insertions, 6 deletions

diff --git a/doc/FAQ b/doc/FAQ
index b7ccc62b069..e74cf8e30a1 100644
--- a/doc/FAQ
+++ b/doc/FAQ
@@ -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?