diff options
author | Bruce Momjian | 2005-01-30 04:20:52 +0000 |
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committer | Bruce Momjian | 2005-01-30 04:20:52 +0000 |
commit | d79f2d4036da2e5fd022e92c178ea342855db586 (patch) | |
tree | 3fb1437fe51d8dad9302980df782a981d8f77dc9 /doc/FAQ | |
parent | bfc536217db70399a9ee8bfebdbd667390fecc9a (diff) |
Remove IN/slow FAQ item because it only applies to <=7.3.X.
Diffstat (limited to 'doc/FAQ')
-rw-r--r-- | doc/FAQ | 44 |
1 files changed, 11 insertions, 33 deletions
@@ -1,7 +1,7 @@ Frequently Asked Questions (FAQ) for PostgreSQL - Last updated: Sat Jan 29 23:15:42 EST 2005 + Last updated: Sat Jan 29 23:20:03 EST 2005 Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) @@ -76,13 +76,12 @@ 4.16) Why does my large-object operations get "invalid large obj descriptor"? 4.17) How do I create a column that will default to the current time? - 4.18) Why are my subqueries using IN so slow? - 4.19) How do I perform an outer join? - 4.20) How do I perform queries using multiple databases? - 4.21) How do I return multiple rows or columns from a function? - 4.22) Why can't I reliably create/drop temporary tables in PL/PgSQL + 4.18) How do I perform an outer join? + 4.19) How do I perform queries using multiple databases? + 4.20) How do I return multiple rows or columns from a function? + 4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL functions? - 4.23) What encryption options are available? + 4.22) What encryption options are available? Extending PostgreSQL @@ -922,28 +921,7 @@ BYTEA bytea variable-length byte array (null-byte safe) Use CURRENT_TIMESTAMP: CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP ); - 4.18) Why are my subqueries using IN so slow? - - In versions prior to 7.4, subqueries were joined to outer queries by - sequentially 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 col IN (SELECT subcol FROM subtab); - - to: - SELECT * - FROM tab - WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col); - - For this to be fast, subcol should be an indexed column. - - In version 7.4 and later, IN actually uses the same sophisticated join - techniques as normal queries, and is prefered to using EXISTS. - - 4.19) How do I perform an outer join? + 4.18) How do I perform an outer join? PostgreSQL supports outer joins using the SQL standard syntax. Here are two examples: @@ -973,7 +951,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP ); WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2) ORDER BY col1 - 4.20) How do I perform queries using multiple databases? + 4.19) How do I perform queries using multiple databases? There is no way to query a database other than the current one. Because PostgreSQL loads database-specific system catalogs, it is @@ -983,12 +961,12 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP ); course, a client can make simultaneous connections to different databases and merge the results on the client side. - 4.21) How do I return multiple rows or columns from a function? + 4.20) How do I return multiple rows or columns from a function? In 7.3, you can easily return multiple rows or columns from a function, http://techdocs.postgresql.org/guides/SetReturningFunctions. - 4.22) Why can't I reliably create/drop temporary tables in PL/PgSQL + 4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL functions? PL/PgSQL caches function contents, and an unfortunate side effect is @@ -999,7 +977,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP ); table access in PL/PgSQL. This will cause the query to be reparsed every time. - 4.23) What encryption options are available? + 4.22) What encryption options are available? * contrib/pgcrypto contains many encryption functions for use in SQL queries. |