This patch adds the usage count statistic to the information available in contrib/pgbuffercache. Earlier this month a discussion about my first attempt to instrument the background writer had Tom asking for details about the usage histogram I was seeing, and this patch proved to be the easiest way I found to take a look at that.

In situations where one is trying to optimize the background writer, it's very hard to adjust how much to rely on the LRU writer versus the one that writes everything unless you know whether your dirty buffers are typically used heavily (like index blocks) or not (like new INSERT data). Some statistics about the usage counts in your buffer cache are extremely helpful in making that decision.

I'll even pass along an ugly but fun query that utilizes this. The following will give you a summary of your buffer cache broken into 32 sections. Each line shows the average usage count of that section, as a positive number if most buffers dirty and a negative one if most are clean. If you refresh this frequently enough, you can actually watch things like how checkpoints move through the buffer cache:

SELECT current_timestamp,
-- Split into 32 bins of data
round(bufferid / (cast((select setting from pg_settings where name='shared_buffers') as int) / (32 - 1.0)))
as section, round(
-- Average usage count, capped at 5
case when avg(usagecount)>5 then 5 else avg(usagecount) end *
-- -1 when the majority are clean records, 1 when most are dirty
(case when sum(case when isdirty then 1 else -1 end)>0 then 1 else -1 end)) as color_intensity
FROM pg_buffercache GROUP BY
round(bufferid / (cast((select setting from pg_settings where name='shared_buffers') as int) / (32 - 1.0)));

The 32 can be changed to anything, that's just what fits on my screen. The main idea of the above is that if you dump all this to a file regularly, it's possible to produce a graph of it showing how the cache has changed over time by assigning a different color intensity based on the usage count--at a massive cost in overhead, of course. I'll be passing along all that code once I get it ready for other people to use.

--
* Greg Smith [EMAIL PROTECTED] http://www.gregsmith.com Baltimore, MD
Index: README.pg_buffercache
===================================================================
RCS file: /projects/cvsroot/pgsql/contrib/pg_buffercache/README.pg_buffercache,v
retrieving revision 1.3
diff -c -r1.3 README.pg_buffercache
*** README.pg_buffercache       26 Apr 2006 22:50:17 -0000      1.3
--- README.pg_buffercache       1 Apr 2007 02:17:15 -0000
***************
*** 40,46 ****
     reldatabase    | pg_database.oid      | Database for the relation.
     relblocknumber |                      | Offset of the page in the relation.
     isdirty        |                      | Is the page dirty?
! 
  
    There is one row for each buffer in the shared cache. Unused buffers are
    shown with all fields null except bufferid.
--- 40,46 ----
     reldatabase    | pg_database.oid      | Database for the relation.
     relblocknumber |                      | Offset of the page in the relation.
     isdirty        |                      | Is the page dirty?
!    usagecount     |                      | Page LRU count
  
    There is one row for each buffer in the shared cache. Unused buffers are
    shown with all fields null except bufferid.
***************
*** 60,79 ****
  
    regression=# \d pg_buffercache;
         View "public.pg_buffercache"
!        Column     |  Type   | Modifiers
!   ----------------+---------+-----------
!    bufferid       | integer |
!    relfilenode    | oid     |
!    reltablespace  | oid     |
!    reldatabase    | oid     |
!    relblocknumber | bigint  |
!    isdirty        | boolean |
    View definition:
     SELECT p.bufferid, p.relfilenode, p.reltablespace, p.reldatabase, 
!           p.relblocknumber, p.isdirty
       FROM pg_buffercache_pages() p(bufferid integer, relfilenode oid, 
       reltablespace oid, reldatabase oid, relblocknumber bigint, 
!      isdirty boolean);
  
    regression=# SELECT c.relname, count(*) AS buffers
                 FROM pg_class c INNER JOIN pg_buffercache b
--- 60,81 ----
  
    regression=# \d pg_buffercache;
         View "public.pg_buffercache"
!        Column     |  Type    | Modifiers
!   ----------------+----------+-----------
!    bufferid       | integer  |
!    relfilenode    | oid      |
!    reltablespace  | oid      |
!    reldatabase    | oid      |
!    relblocknumber | bigint   |
!    isdirty        | boolean  |
!    usagecount     | smallint |
! 
    View definition:
     SELECT p.bufferid, p.relfilenode, p.reltablespace, p.reldatabase, 
!           p.relblocknumber, p.isdirty, p.usagecount
       FROM pg_buffercache_pages() p(bufferid integer, relfilenode oid, 
       reltablespace oid, reldatabase oid, relblocknumber bigint, 
!      isdirty boolean, usagecount smallint);
  
    regression=# SELECT c.relname, count(*) AS buffers
                 FROM pg_class c INNER JOIN pg_buffercache b
Index: pg_buffercache.sql.in
===================================================================
RCS file: /projects/cvsroot/pgsql/contrib/pg_buffercache/pg_buffercache.sql.in,v
retrieving revision 1.4
diff -c -r1.4 pg_buffercache.sql.in
*** pg_buffercache.sql.in       26 Apr 2006 22:50:17 -0000      1.4
--- pg_buffercache.sql.in       1 Apr 2007 02:17:15 -0000
***************
*** 12,18 ****
  CREATE VIEW pg_buffercache AS
        SELECT P.* FROM pg_buffercache_pages() AS P
        (bufferid integer, relfilenode oid, reltablespace oid, reldatabase oid, 
!        relblocknumber int8, isdirty bool);
   
  -- Don't want these to be available at public.
  REVOKE ALL ON FUNCTION pg_buffercache_pages() FROM PUBLIC;
--- 12,18 ----
  CREATE VIEW pg_buffercache AS
        SELECT P.* FROM pg_buffercache_pages() AS P
        (bufferid integer, relfilenode oid, reltablespace oid, reldatabase oid, 
!        relblocknumber int8, isdirty bool, usagecount int2);
   
  -- Don't want these to be available at public.
  REVOKE ALL ON FUNCTION pg_buffercache_pages() FROM PUBLIC;
Index: pg_buffercache_pages.c
===================================================================
RCS file: 
/projects/cvsroot/pgsql/contrib/pg_buffercache/pg_buffercache_pages.c,v
retrieving revision 1.11
diff -c -r1.11 pg_buffercache_pages.c
*** pg_buffercache_pages.c      22 Oct 2006 17:49:21 -0000      1.11
--- pg_buffercache_pages.c      1 Apr 2007 02:17:15 -0000
***************
*** 16,22 ****
  #include "utils/relcache.h"
  
  
! #define NUM_BUFFERCACHE_PAGES_ELEM    6
  
  PG_MODULE_MAGIC;
  
--- 16,22 ----
  #include "utils/relcache.h"
  
  
! #define NUM_BUFFERCACHE_PAGES_ELEM    7
  
  PG_MODULE_MAGIC;
  
***************
*** 35,40 ****
--- 35,41 ----
        BlockNumber blocknum;
        bool            isvalid;
        bool            isdirty;
+       uint16          usagecount;
  }     BufferCachePagesRec;
  
  
***************
*** 91,96 ****
--- 92,99 ----
                                                   INT8OID, -1, 0);
                TupleDescInitEntry(tupledesc, (AttrNumber) 6, "isdirty",
                                                   BOOLOID, -1, 0);
+               TupleDescInitEntry(tupledesc, (AttrNumber) 7, "usage_count",
+                                                  INT2OID, -1, 0);
  
                fctx->tupdesc = BlessTupleDesc(tupledesc);
  
***************
*** 126,131 ****
--- 129,135 ----
                        fctx->record[i].reltablespace = 
bufHdr->tag.rnode.spcNode;
                        fctx->record[i].reldatabase = bufHdr->tag.rnode.dbNode;
                        fctx->record[i].blocknum = bufHdr->tag.blockNum;
+                       fctx->record[i].usagecount = bufHdr->usage_count;
  
                        if (bufHdr->flags & BM_DIRTY)
                                fctx->record[i].isdirty = true;
***************
*** 172,177 ****
--- 176,182 ----
                        nulls[3] = true;
                        nulls[4] = true;
                        nulls[5] = true;
+                       nulls[6] = true;
                }
                else
                {
***************
*** 185,190 ****
--- 190,197 ----
                        nulls[4] = false;
                        values[5] = BoolGetDatum(fctx->record[i].isdirty);
                        nulls[5] = false;
+                       values[6] = Int16GetDatum(fctx->record[i].usagecount);
+                       nulls[6] = false;
                }
  
                /* Build and return the tuple. */
---------------------------(end of broadcast)---------------------------
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