Update IN/EXISTS item.

diff --git a/doc/FAQ b/doc/FAQ
index b7ccc62b0695fea08e355ceb211e0947ff107348..e74cf8e30a114ddbf7d387bbc4ce2e4dd6ff916d 100644 (file)
--- 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 ([email protected])
    
@@ -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?
     

diff --git a/doc/src/FAQ/FAQ.html b/doc/src/FAQ/FAQ.html
index cad83373cf8c8aeb471cc79dda08a4e8a365c1db..54587662260127d91c55db5def960a1169b935d6 100644 (file)
--- a/doc/src/FAQ/FAQ.html
+++ b/doc/src/FAQ/FAQ.html
@@ -14,7 +14,7 @@
   alink="#0000ff">
     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 (
     "mailto:[email protected]">[email protected])
@@ -1282,22 +1282,25 @@ BYTEA           bytea           variable-length byte array (null-byte safe)
 
     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
+    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)
 
 
 
+    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?