We have a large application written in C language (not C++) using embedded SQL. The number of tables in the database is about 60, with about 80 SQL indexes. Total number of SQL statements is about 800 to 1000. All SQL statements are generated by a program generator handling the differences between Oracle/Informix/PostgreSQL. All SELECT statements are "simple", extracting data from only one table (no joins or similar).
The application works under Oracle (using Pro-C preprocessor) and Informix (using ESQL preprocessor). Under PostgreSQL we use the ECPG preprocessor. FYI it took us only a few weeks to port the application to PostgreSQL.
The application works under all these database systems and has been trimmed with the necessary SQL indexes etc. For the PostgreSQL we continually run the necessary VACUUM + ANALYZE to ensure the PostgreSQL optimizer uses the proper SQL indexes (there are no table scans).
The documentation for PostgreSQL 7.3 explains about introducing the optimized PREPARE/EXECUTE statements, but we have discovered this is not implemented (yet) in ECPG (we are using release 7.3.2). See comments in "ecpg/preproc/preproc.y".
We have a major installation currently using Oracle, but they want to switch to PostgreSQL when the access time reaches a reasonable level. The size of database (tables + indexes) is about 120 GB, and the number of SELECTs from the tables is of the magnitude 200 to 500 million per day, split over a number of SUN E10K machines (about 30 processors each with 1 GB RAM). (The Oracle license and annual maintenance fee on this setup are exorbitant!!)
Without the optimized PREPARE/EXECUTE we see a SELECT time of about 30 milliseconds (measured on the SUN E10K) of which most seems to be CPU time spent inside postmaster. The similar time from Oracle is about 1 to 5 milliseconds on the same hardware. Both PostgreSQL and Oracle have been setup with a cache (shared_buffers) of 120000.
To our customer the 30 milliseconds are not acceptable. Based on the PostgreSQL 7.3 documentation we should expect "improved performance" when using prepared queries.
This leads to a couple of questions regarding the optimized PREPARE/EXECUTE:
- Do you have any indication of the performance improvement (measured in percentage)?
- When will the optimized PREPARE/EXECUTE be implemented in ECPG?
Best Regards
Bo Tveden
Mads Madsen
BTS Solutions
