On Nov 20, 2007 8:10 AM, Filip Rembiałkowski <plk.zuber@xxxxxxxxx> wrote:
> 2007/11/20, Josh Harrison <joshques@xxxxxxxxx>:
> > We are working on migrating our database from oracle to postgres.
> > Postgres tablesize is twice than oracle tablesize for all my
> > tables.
> Interesting. Which postgresql version?
Version 8.2.3
> >And so the query also takes twice as much time than oracle.
> This is even more interesting :) What query? can you show it here
> along with EXPLAIN ANALYZE?
explain analyze select count(*) from dummy1
QUERY PLAN
-------------------------------
Aggregate (cost=1192999.60..1192999.61 rows=1 width=0) (actual
time=109792.239..109792.239 rows=1 loops=1)
-> Seq Scan on dummy1 (cost=0.00..1119539.48 rows=29384048
30000000 width=0) (actual time=0.027..101428.016 rows=29384048
loops=1)
Total runtime: 109792.332 ms
Postgresql takes 1m 40s for this query
Oracle takes 45 sec
It is just a count(*) query. I know count(*) query is slower in
postgres becoz it doesn't use index. But in Oracle I give the query as
select /*+full(dummy1)*/ count(*) from dummy1
with the hint so that oracle uses full table scan and not the index scan.
>
> Did you do some index tuning or do you just expect the indexes ported
> from Oracle schema to work?
I created the indexes and Im not sure what kind of tuning neds to be
done for the indexes. But this above query doesnt use any indexes.
> Did you run ANALYZE after populating database?
Yes
> What are server parameters and did you tune postgres config to fit them?
I had attached my config file and the table structure
> > So
> > we were checking to see what makes postgres slower than oracle even
> > for basic full tablescan queries.
> I'm curious too :) please let me know if you resolve this mystery :)
>
>
> > There were a couple of things we noted.
> > 1. Tablesize twice as much than oracle-- Im not sure if postgres null
> > columns has any overhead since we have lots of null columns in our
> > tables.Does postgresql has lots of overhead for null columns?
> I've expained this previously - you have a bitmap in each tuple.
> Bitmap size is (NATTS+7) % 8
>
> > 2. Oracle seems to be reading larger bocks than postgresql (when we
> > examined the iostat and vmstat) (we had set postgres' db block size as
> > 8 and oracle's is 16kb...)
> yes, 8 kB is default pg block size. it is not recommended to change it
> - however it could be useful in some situations - but I doubt it would
> speedup your queries twice, whatever they are.
>
>
>
> --
> Filip Rembiałkowski
>
Thanks again
josh
# -----------------------------
# PostgreSQL configuration file
# -----------------------------
#
# This file consists of lines of the form:
#
# name = value
#
# (The '=' is optional.) White space may be used. Comments are introduced
# with '#' anywhere on a line. The complete list of option names and
# allowed values can be found in the PostgreSQL documentation. The
# commented-out settings shown in this file represent the default values.
#
# Please note that re-commenting a setting is NOT sufficient to revert it
# to the default value, unless you restart the postmaster.
#
# Any option can also be given as a command line switch to the
# postmaster, e.g. 'postmaster -c log_connections=on'. Some options
# can be changed at run-time with the 'SET' SQL command.
#
# This file is read on postmaster startup and when the postmaster
# receives a SIGHUP. If you edit the file on a running system, you have
# to SIGHUP the postmaster for the changes to take effect, or use
# "pg_ctl reload". Some settings, such as listen_addresses, require
# a postmaster shutdown and restart to take effect.
#---------------------------------------------------------------------------
# FILE LOCATIONS
#---------------------------------------------------------------------------
# The default values of these variables are driven from the -D command line
# switch or PGDATA environment variable, represented here as ConfigDir.
#data_directory = 'ConfigDir' # use data in another directory
#hba_file = 'ConfigDir/pg_hba.conf' # host-based authentication file
#ident_file = 'ConfigDir/pg_ident.conf' # IDENT configuration file
# If external_pid_file is not explicitly set, no extra pid file is written.
#external_pid_file = '(none)' # write an extra pid file
#---------------------------------------------------------------------------
# CONNECTIONS AND AUTHENTICATION
#---------------------------------------------------------------------------
# - Connection Settings -
#tcpip_socket=true
listen_addresses = '*' # what IP address(es) to listen on;
# comma-separated list of addresses;
# defaults to 'localhost', '*' = all
#port = 5432
max_connections = 100
# note: increasing max_connections costs ~400 bytes of shared memory per
# connection slot, plus lock space (see max_locks_per_transaction). You
# might also need to raise shared_buffers to support more connections.
#superuser_reserved_connections = 2
#unix_socket_directory = ''
#unix_socket_group = ''
#unix_socket_permissions = 0777 # octal
#bonjour_name = '' # defaults to the computer name
# - Security & Authentication -
#authentication_timeout = 60 # 1-600, in seconds
#ssl = off
#password_encryption = on
#db_user_namespace = off
# Kerberos
#krb_server_keyfile = ''
#krb_srvname = 'postgres'
#krb_server_hostname = '' # empty string matches any keytab entry
#krb_caseins_users = off
# - TCP Keepalives -
# see 'man 7 tcp' for details
#tcp_keepalives_idle = 0 # TCP_KEEPIDLE, in seconds;
# 0 selects the system default
#tcp_keepalives_interval = 0 # TCP_KEEPINTVL, in seconds;
# 0 selects the system default
#tcp_keepalives_count = 0 # TCP_KEEPCNT;
# 0 selects the system default
#---------------------------------------------------------------------------
# RESOURCE USAGE (except WAL)
#---------------------------------------------------------------------------
# - Memory -
shared_buffers =50000 # min 16 or max_connections*2, 8KB each
temp_buffers = 5000 # min 100, 8KB each
#max_prepared_transactions = 5 # can be 0 or more
# note: increasing max_prepared_transactions costs ~600 bytes of shared memory
# per transaction slot, plus lock space (see max_locks_per_transaction).
work_mem = 16384 # min 64, size in KB
maintenance_work_mem = 262144 # min 1024, size in KB
#max_stack_depth = 2048 # min 100, size in KB
# - Free Space Map -
#max_fsm_pages = 20000 # min max_fsm_relations*16, 6 bytes each
#max_fsm_relations = 1000 # min 100, ~70 bytes each
# - Kernel Resource Usage -
#max_files_per_process = 1000 # min 25
#preload_libraries = ''
# - Cost-Based Vacuum Delay -
vacuum_cost_delay = 50 # 0-1000 milliseconds
#vacuum_cost_page_hit = 1 # 0-10000 credits
#vacuum_cost_page_miss = 10 # 0-10000 credits
#vacuum_cost_page_dirty = 20 # 0-10000 credits
#vacuum_cost_limit = 200 # 0-10000 credits
# - Background writer -
#bgwriter_delay = 200 # 10-10000 milliseconds between rounds
#bgwriter_lru_percent = 1.0 # 0-100% of LRU buffers scanned/round
#bgwriter_lru_maxpages = 5 # 0-1000 buffers max written/round
#bgwriter_all_percent = 0.333 # 0-100% of all buffers scanned/round
#bgwriter_all_maxpages = 5 # 0-1000 buffers max written/round
#---------------------------------------------------------------------------
# WRITE AHEAD LOG
#---------------------------------------------------------------------------
# - Settings -
#fsync = on # turns forced synchronization on or off
#wal_sync_method = fsync # the default is the first option
# supported by the operating system:
# open_datasync
# fdatasync
# fsync
# fsync_writethrough
# open_sync
#full_page_writes = on # recover from partial page writes
#wal_buffers = 8 # min 4, 8KB each
#commit_delay = 0 # range 0-100000, in microseconds
#commit_siblings = 5 # range 1-1000
# - Checkpoints -
#checkpoint_segments = 3 # in logfile segments, min 1, 16MB each
#checkpoint_timeout = 300 # range 30-3600, in seconds
#checkpoint_warning = 30 # in seconds, 0 is off
# - Archiving -
#archive_command = '' # command to use to archive a logfile
# segment
#---------------------------------------------------------------------------
# QUERY TUNING
#---------------------------------------------------------------------------
# - Planner Method Configuration -
#enable_bitmapscan = on
#enable_hashagg = on
#enable_hashjoin = on
#enable_indexscan = on
#enable_mergejoin = on
#enable_nestloop = on
#enable_seqscan = on
#enable_sort = on
#enable_tidscan = on
#constraint_exclusion=on
# - Planner Cost Constants -
effective_cache_size = 300000 # typically 8KB each
random_page_cost = 4 # units are one sequential page fetch
# cost
#cpu_tuple_cost = 0.01 # (same)
#cpu_index_tuple_cost = 0.001 # (same)
#cpu_operator_cost = 0.0025 # (same)
# - Genetic Query Optimizer -
geqo = on
geqo_threshold = 14
#geqo_effort = 5 # range 1-10
#geqo_pool_size = 0 # selects default based on effort
#geqo_generations = 0 # selects default based on effort
#geqo_selection_bias = 2.0 # range 1.5-2.0
# - Other Planner Options -
#default_statistics_target = 10 # range 1-1000
#constraint_exclusion = off
#from_collapse_limit = 8
#join_collapse_limit = 8 # 1 disables collapsing of explicit
# JOINs
#---------------------------------------------------------------------------
# ERROR REPORTING AND LOGGING
#---------------------------------------------------------------------------
# - Where to Log -
#log_destination = 'stderr' # Valid values are combinations of
# stderr, syslog and eventlog,
# depending on platform.
# This is used when logging to stderr:
redirect_stderr = on # Enable capturing of stderr into log
# files
# These are only used if redirect_stderr is on:
#log_directory = 'pg_log' # Directory where log files are written
# Can be absolute or relative to PGDATA
#log_filename = 'postgresql-%Y-%m-%d_%H%M%S.log' # Log file name pattern.
# Can include strftime() escapes
log_truncate_on_rotation = on # If on, any existing log file of the same
# name as the new log file will be
# truncated rather than appended to. But
# such truncation only occurs on
# time-driven rotation, not on restarts
# or size-driven rotation. Default is
# off, meaning append to existing files
# in all cases.
#log_rotation_age = 1440 # Automatic rotation of logfiles will
# happen after so many minutes. 0 to
# disable.
#log_rotation_size = 10240 # Automatic rotation of logfiles will
# happen after so many kilobytes of log
# output. 0 to disable.
# These are relevant when logging to syslog:
#syslog_facility = 'LOCAL0'
#syslog_ident = 'postgres'
# - When to Log -
client_min_messages = notice # Values, in order of decreasing detail:
# debug5
# debug4
# debug3
# debug2
# debug1
# log
# notice
# warning
# error
log_min_messages = notice # Values, in order of decreasing detail:
# debug5
# debug4
# debug3
# debug2
# debug1
# info
# notice
# warning
# error
# log
# fatal
# panic
log_error_verbosity = verbose # terse, default, or verbose messages
log_min_error_statement = notice # Values in order of increasing severity:
# debug5
# debug4
# debug3
# debug2
# debug1
# info
# notice
# warning
# error
# panic(off)
log_min_duration_statement = -1 # -1 is disabled, 0 logs all statements
# and their durations, in milliseconds.
#silent_mode = off # DO NOT USE without syslog or
# redirect_stderr
# - What to Log -
debug_print_parse = off
debug_print_rewritten = off
debug_print_plan = off
debug_pretty_print = on
log_connections = on
log_disconnections = on
log_duration = on
log_line_prefix = '%h:%u@%d %t %i' # Special values:
# %u = user name
# %d = database name
# %r = remote host and port
# %h = remote host
# %p = PID
# %t = timestamp (no milliseconds)
# %m = timestamp with milliseconds
# %i = command tag
# %c = session id
# %l = session line number
# %s = session start timestamp
# %x = transaction id
# %q = stop here in non-session
# processes
# %% = '%'
# e.g. '<%u%%%d> '
log_statement = 'none' # none, mod, ddl, all
log_hostname = off
#---------------------------------------------------------------------------
# RUNTIME STATISTICS
#---------------------------------------------------------------------------
# - Statistics Monitoring -
#log_parser_stats = off
#log_planner_stats = off
#log_executor_stats = off
log_statement_stats = on
# - Query/Index Statistics Collector -
stats_start_collector = on
stats_command_string = on
stats_block_level = on
stats_row_level = on
#stats_reset_on_server_start = off
#---------------------------------------------------------------------------
# AUTOVACUUM PARAMETERS
#---------------------------------------------------------------------------
autovacuum = on # enable autovacuum subprocess?
#autovacuum_naptime = 60 # time between autovacuum runs, in secs
#autovacuum_vacuum_threshold = 1000 # min # of tuple updates before
# vacuum
#autovacuum_analyze_threshold = 500 # min # of tuple updates before
# analyze
#autovacuum_vacuum_scale_factor = 0.4 # fraction of rel size before
# vacuum
#autovacuum_analyze_scale_factor = 0.2 # fraction of rel size before
# analyze
#autovacuum_vacuum_cost_delay = -1 # default vacuum cost delay for
# autovac, -1 means use
# vacuum_cost_delay
#autovacuum_vacuum_cost_limit = -1 # default vacuum cost limit for
# autovac, -1 means use
# vacuum_cost_limit
#---------------------------------------------------------------------------
# CLIENT CONNECTION DEFAULTS
#---------------------------------------------------------------------------
# - Statement Behavior -
search_path = '$user,mw,public' # schema names
#default_tablespace = '' # a tablespace name, '' uses
# the default
#check_function_bodies = on
#default_transaction_isolation = 'read committed'
#default_transaction_read_only = off
#statement_timeout = 0 # 0 is disabled, in milliseconds
# - Locale and Formatting -
#datestyle = 'iso, mdy'
#timezone = unknown # actually, defaults to TZ
# environment setting
#australian_timezones = off
#extra_float_digits = 0 # min -15, max 2
#client_encoding = utf8 # actually, defaults to database
# encoding
# These settings are initialized by initdb -- they might be changed
lc_messages = 'C' # locale for system error message
# strings
lc_monetary = 'en_CA.ISO8859-1' # locale for monetary formatting
lc_numeric = 'en_CA.ISO8859-1' # locale for number formatting
lc_time = 'C' # locale for time formatting
# - Other Defaults -
#explain_pretty_print = on
#dynamic_library_path = '$libdir'
#---------------------------------------------------------------------------
# LOCK MANAGEMENT
#---------------------------------------------------------------------------
#deadlock_timeout = 1000 # in milliseconds
#max_locks_per_transaction = 64 # min 10
# note: each lock table slot uses ~220 bytes of shared memory, and there are
# max_locks_per_transaction * (max_connections + max_prepared_transactions)
# lock table slots.
#---------------------------------------------------------------------------
# VERSION/PLATFORM COMPATIBILITY
#---------------------------------------------------------------------------
# - Previous Postgres Versions -
#add_missing_from = off
#backslash_quote = safe_encoding # on, off, or safe_encoding
#default_with_oids = off
#escape_string_warning = off
#regex_flavor = advanced # advanced, extended, or basic
#sql_inheritance = on
# - Other Platforms & Clients -
#transform_null_equals = off
#---------------------------------------------------------------------------
# CUSTOMIZED OPTIONS
#---------------------------------------------------------------------------
#custom_variable_classes = '' # list of custom variable class names
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