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Re: postgresql storage and performance questions

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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
---------------------------(end of broadcast)---------------------------
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