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Many relational databases keep internal statistics about the composition of data in all tables of the system. These provide information about the total number of values in each column as well as their distribution, among other details. The database query optimizer leverages these statistics to construct the potentially best query strategy, including the order in which to query joined tables or the order in which to apply selection criteria. Depending on database configuration, these statistics may be updated automatically, e.g. after table content is updated, periodically (e.g. each night) or manually. Out-of-date statistics, which do no longer reflect the current content of a table, may lead to notable performance impacts because of inappropriate query strategies proposed by the optimizer. Thus, unless the database system is generally configured to update table statistics immediately after loading, it is recommended to include a procedure to explicitly update table statistics at the end of an i2b2 ETL pathway.

Wiki MarkupThe following Oracle PL/SQL script invokes the function to gather the statistics of the i2b2 schema. The placeholder \The placeholder [SCHEMA\] needs to be replaced with the name of the schema containing the i2b2 fact and/or dimension tables.

BEGIN
DBMS_STATS.GATHER_SCHEMA_STATS([SCHEMA]);
END;

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