Drop Down MenusCSS Drop Down MenuPure CSS Dropdown Menu

Postgresql Increase Performance Query (Sequential scan) Without Index -understanding For Cache Memory

-
  • A sequential scan i.e when there is no index and postgres has to fetch all the data from disk are a problem area for a cache like this.
  • Since a single seq scan can wipe all of the data from a cache, it is handled differently.
  • Instead of using a normal LRU/clock sweep algorithm, it uses a series of buffers of total 256 K.B in size. The below plan shows how it is handled.
test=# explain (analyze,buffers) select count(*) from emp;
   
  Aggregate  (cost=48214.95..48214.96 rows=1 width=0) (actual time=3874.445..3874.445 rows=1 loops=1)
    Buffers: shared read=35715
    ->  Seq Scan on emp  (cost=0.00..45714.96 rows=999996 width=0) (actual time=6.024..3526.606 rows=1000000 loops=1)
          Buffers: shared read=35715
  Planning time: 0.114 ms
  Execution time: 3874.509 ms

Executing the above query again.

test=# explain (analyze,buffers) select count(*) from emp;
      Aggregate  (cost=48214.95..48214.96 rows=1 width=0) (actual time=426.385..426.385 rows=1 loops=1)
    Buffers: shared hit=32 read=35683
    ->  Seq Scan on emp  (cost=0.00..45714.96 rows=999996 width=0) (actual time=0.036..285.363 rows=1000000 loops=1)
          Buffers: shared hit=32 read=35683
  Planning time: 0.048 ms
  Execution time: 426.431 ms
We can see that exactly 32 blocks have moved into memory i.e 32 * 8 = 256 KB.

Comments

Post a Comment

Popular posts from this blog

7 Steps to configure BDR replication in postgresql

-
Image
 The  BDR  (Bi-Directional Replication) project adds multi-master replication to PostgreSQL 9.4.  Postgres-BDR has a lower impact on the masters(s) than trigger-based replication solutions. There is no write-amplification, as it does not require triggers to write to queue tables in order to replicate writes.   Here We are using  postgres version 9.4.12 and bdr version 1.0.2. for configuring  multi master replication . Simply Following 7 steps you can configure the multi master replication in postgresql. To download the bdr in below link. .   https://github.com/2ndQuadrant/bdr/archive/bdr-pg/REL9_4_12-1.tar.gz $ tar -xzvf REL9_4_12-1.tar.gz $ wget https://github.com/2ndQuadrant/bdr/archive/bdr-plugin/1.0.2.tar.gz $ tar -xzvf 1.0.2.tar.gz 1. To install BDR. $ cd ~/bdr-bdr-pg-REL9_4_12-1 $ ./configure --prefix=/usr/lib/postgresql/9.4 --enable-debug --with-openssl $ make -j4 -s install-world $ cd ~/bdr-bdr-plugin-1.0.2 $ PATH=/usr/lib/postgresql/9.4/bin:"$PATH" ./c
Read more

How to find the server is whether standby (slave) or primary(master) in Postgresql replication ?

-
Method 1 You can check the mode of the server using "pg_controldata". [pgsql@test~]$ pg_controldata /usr/local/pgsql/data84/ Database cluster state: in archive recovery --> This is Standby Database Database cluster state: in production --> This is Production Database [Master] Method 2 You can use pg_is_in_recovery() which returns True if recovery is still in progress(so the server is running in standby mode or slave) postgres=# select pg_is_in_recovery(); pg_is_in_recovery ------------------- t (1 row) If Return    false   so the server is running in primary mode or master postgres=# select pg_is_in_recovery(); pg_is_in_recovery ------------------- f (1 row)
Read more

apt-add-repository

-
apt-add-repository -V (return code: 2) Usage: apt-add-repository <sourceline> apt-add-repository is a script for adding apt sources.list entries. It can be used to add any repository and also provides a shorthand syntax for adding a Launchpad PPA (Personal Package Archive) repository. <sourceline> - The apt repository source line to add. This is one of: a complete apt line in quotes, a repo url and areas in quotes (areas defaults to 'main') a PPA shortcut. a distro component Examples: apt-add-repository 'deb http://myserver/path/to/repo stable myrepo' apt-add-repository 'http://myserver/path/to/repo myrepo' apt-add-repository 'https://packages.medibuntu.org free non-free' apt-add-repository http://extras.ubuntu.com/ubuntu apt-add-repository ppa:user/repository apt-add-repository ppa:user/distro/repository apt-add-repository multiverse If --remove is given the tool will remove the given sourceline fr
Read more

How to Get Table Size, Database Size, Indexes Size, schema Size, Tablespace Size, column Size in PostgreSQL Database

-
In this post, I am sharing few important function for finding the size of database, table and index in PostgreSQL. Finding object size in postgresql database is very important and common. Is it very useful to know the exact size occupied by the object at the tablespace. The object size in the following scripts is in GB. The scripts have been formatted to work very easily with PUTTY SQL Editor. 1. Checking table size excluding table dependency: SELECT pg_size_pretty(pg_relation_size('mhrordhu_shk.mut_kharedi_audit')); pg_size_pretty ---------------- 238 MB (1 row) 2. Checking table size including table dependency: SELECT pg_size_pretty(pg_total_relation_size('mhrordhu_shk.mut_kharedi_audit')); pg_size_pretty ---------------- 268 MB (1 row) 3. Finding individual postgresql database size SELECT pg_size_pretty(pg_database_size('db_name')); 4. Finding individual table size for postgresql database -including dependency index: SELECT pg_size_pretty(pg_total_rel
Read more

ORA-01261: Parameter db_recovery_file_dest destination string cannot be translated ORA-01262: Stat failed on a file destination directory Linux-x86_64 Error: 2: No such file or directory

-
While I start my database with my initialization parameter it fails with oracle error Solution :- su oracle run environment variable sqlplus / as sysdba SQL>create pfile from spfile; Then remove or comment below line in pfile db_recovery_file_dest Save and exit su oracle run environment variable sqlplus / as sysdba SQL>startup mount; ORACLE instance started. Total System Global Area 1068937216 bytes Fixed Size 2166536 bytes Variable Size 427819256 bytes Database Buffers 624951296 bytes Redo Buffers 14000128 bytes it should became the mount stage then open the DB as follows SQL>alter database open; Database altered.
Read more