edbs ppas comes with an oracle compatibility layer. in this and some future posts I’ll take a look at what this layer is about and what you can do with it.
there are four parameters which control the behaviour of the oracle compatibility layer:
- edb_redwood_date
- edb_redwood_raw_names
- edb_redwood_strings
- edb_stmt_level_tx
lets check the catalog for these parameters and see if we can change them on the fly or if we need to restart the database server:
select name,setting,context
from pg_settings
where name in ('edb_redwood_date'
,'edb_redwood_raw_names'
,'edb_redwood_strings'
,'edb_stmt_level_tx');
name | setting | context
-----------------------+---------+---------
edb_redwood_date | on | user
edb_redwood_raw_names | off | user
edb_redwood_strings | on | user
edb_stmt_level_tx | off | user
fine, all have the context “user” which means we can change them without restarting the server (btw: the above settings are the default if ppas is installed in oracle compatibility mode).
what is edb_redwood_date about?
in postgres, if you specify a column as date there is no time information. setting this parameter to “on” tells the server to use a timestamp instead of pure date data type whenever date is specified for a column. in oracle a column of type date includes the time component, too.
lets switch it to off for now:
edb=# alter system set edb_redwood_date=off;
ALTER SYSTEM
edb=# select pg_reload_conf();
pg_reload_conf
----------------
t
(1 row)
edb=# select name,setting,context from pg_settings where name = 'edb_redwood_date';
name | setting | context
------------------+---------+---------
edb_redwood_date | off | user
(1 row)
and now lets create a simple table with a date column and insert one row:
edb=# create table t1 ( a date );
CREATE TABLE
edb=# \d t1
Table "enterprisedb.t1"
Column | Type | Modifiers
--------+------+-----------
a | date |
edb=# insert into t1 values (date '2014-01-01');
INSERT 0 1
edb=# select * from t1;
a
-----------
01-JAN-14
(1 row)
no time component available. now switch edb_redwood_date to “on”:
edb=# alter system set edb_redwood_date=on;
ALTER SYSTEM
edb=# select pg_reload_conf();
pg_reload_conf
----------------
t
(1 row)
edb=# select name,setting,context from pg_settings where name = 'edb_redwood_date';
name | setting | context
------------------+---------+---------
edb_redwood_date | on | user
(1 row)
… and create another table with type date for the column and do the same insert:
edb=# create table t2 ( a date );
CREATE TABLE
edb=# insert into t2 values ( date '2014-01-01');
INSERT 0 1
edb=# select * from t2;
a
--------------------
01-JAN-14 00:00:00
(1 row)
here we go. the time component is now included. but how is this possible? the server created the column with type “timestamp (0)” on the fly:
edb=# \d t2
Table "enterprisedb.t2"
Column | Type | Modifiers
--------+-----------------------------+-----------
a | timestamp without time zone |
what is edb_redwood_raw_names about?
when oracle compatibilty mode is used various oracle catalog views are accessible in ppas, e.g;
edb=# select viewname from pg_views where viewname like 'dba%' limit 5;
viewname
-----------------
dba_tables
dba_users
dba_constraints
dba_all_tables
dba_triggers
(5 rows)
now lets create two tables while edb_redwood_raw_names is set to its default (false/off):
edb=# create table TEST1 (A int); CREATE TABLE edb=# create table test2 (a int); CREATE TABLE
both of these tables are displayed in upper case when looking at the oracle catalog views:
edb=# select table_name from dba_tables where table_name in ('TEST1','TEST2');
table_name
------------
TEST1
TEST2
(2 rows)
setting edb_redwood_raw_names to true/on changes this behaviour:
edb=# set edb_redwood_raw_names to on;
SET
edb=# show edb_redwood_raw_names;
edb_redwood_raw_names
-----------------------
on
(1 row)
edb=# create table TEST3 (A int);
CREATE TABLE
edb=# create table test4 (a int);
CREATE TABLE
edb=# select table_name from dba_tables where table_name in ('TEST3','TEST4');
table_name
------------
(0 rows)
edb=# select table_name from dba_tables where table_name in ('test3','test4');
table_name
------------
test3
test4
(2 rows)
what is edb_redwood_strings about?
edb_redwood_strings controls concatenation of strings. in plain postgres, if a string is concatenated with null the result is null:
edb=# show edb_redwood_strings; edb_redwood_strings --------------------- on (1 row) edb=# set edb_redwood_strings to off; SET edb=# show edb_redwood_strings; edb_redwood_strings --------------------- off (1 row) edb=# select 'aaaa'||null; ?column? ---------- (1 row)
in oracle the behaviour is the other way around. if a string is concatenated with null the result is the original string:
edb=# set edb_redwood_strings to on; SET edb=# show edb_redwood_strings; edb_redwood_strings --------------------- on (1 row) edb=# select 'aaaa'||null; ?column? ---------- aaaa (1 row)
what is edb_stmt_level_tx about?
this is all about “statement level transaction isolation”, which is the default behaviour in oracle. lets set up a little test case to demonstrate this:
edb=# create table t1 ( a int, b varchar(3) );
CREATE TABLE
edb=# alter table t1 add constraint chk_b check ( b in ('aaa','bbb'));
ALTER TABLE
edb=# create table t2 ( c int, d date );
CREATE TABLE
edb=# alter table t2 add constraint chk_c check ( c in (1,2,3));
ALTER TABLE
the default setting for edb_stmt_level_tx is off:
edb=# show edb_stmt_level_tx; edb_stmt_level_tx ------------------- off (1 row)
lets insert some rows in the tables and let the last insert fail (autocommit needs to be off as each statement commits automatically otherwise):
edb=# set autocommit off; edb=# show autocommit; autocommit OFF edb=# insert into t1 (a,b) values (1,'aaa'); INSERT 0 1 edb=# insert into t1 (a,b) values (2,'aaa'); INSERT 0 1 edb=# insert into t2 (c,d) values (1,sysdate); INSERT 0 1 edb=# insert into t2 (c,d) values (5,sysdate); ERROR: new row for relation "t2" violates check constraint "chk_c" DETAIL: Failing row contains (5, 24-FEB-15 10:52:29).
if we now do a commit, which rows are there?
edb=# commit; ROLLBACK edb=# select * from t1; a | b ---+--- (0 rows) edb=# select * from t2; c | d ---+--- (0 rows)
all gone (notice the ROLLBACK after the commit statement). lets witch edb_stmt_level_tx to on and repeat the test:
edb=# set edb_stmt_level_tx to on; SET edb=# show edb_stmt_level_tx; edb_stmt_level_tx ------------------- on (1 row) edb=# show autocommit; autocommit OFF edb=# insert into t1 (a,b) values (1,'aaa'); INSERT 0 1 edb=# insert into t1 (a,b) values (2,'aaa'); INSERT 0 1 edb=# insert into t2 (c,d) values (1,sysdate); INSERT 0 1 edb=# insert into t2 (c,d) values (5,sysdate); ERROR: new row for relation "t2" violates check constraint "chk_c" DETAIL: Failing row contains (5, 24-FEB-15 10:55:52). edb=# commit; COMMIT edb=# select * from t1; a | b ---+----- 1 | aaa 2 | aaa (2 rows) edb=# select * from t2; c | d ---+-------------------- 1 | 24-FEB-15 10:55:49 (1 row)
now all the rows are there except for the failing one. this is what edb_stmt_level_tx is about. by default postgresql rolls back everything since the start of the transaction. when switching edb_stmt_level_tx to on only the failing statement is rolled back. in addition, if edb_stmt_level_tx is set to off you can not continue the transaction until either commit or rollback is issued:
edb=# set edb_stmt_level_tx to off; SET edb=# insert into t1 (a,b) values (1,'aaa'); INSERT 0 1 edb=# insert into t1 (a,b) values (2,'aaa'); INSERT 0 1 edb=# insert into t2 (c,d) values (1,sysdate); INSERT 0 1 edb=# insert into t2 (c,d) values (5,sysdate); ERROR: new row for relation "t2" violates check constraint "chk_c" DETAIL: Failing row contains (5, 24-FEB-15 11:06:52). edb=# insert into t2 (c,d) values (2,sysdate); ERROR: current transaction is aborted, commands ignored until end of transaction block
pmatop
metric selection
real time graphs
Daniel Westermann's Blog 