If you remove rows from a table, you can insert removed IDs explicitly, it will not have any effect on the sequence generator. Continue, now it will use ID 9 INSERT INTO teams (name ) VALUES ( 'Newcastle United' ) ERROR: duplicate key value violates unique constraint "teams_id_key" - DETAIL: Key (id)=(8) already exists. Will try to assign ID 8 that already inserted INSERT INTO teams (name ) VALUES ( 'Some team' ) You can get an error if there is an UNIQUE constraint, or duplicate IDs can be inserted: Note that the sequence generator may have conflicts with IDs that were already inserted using explicit values. Continue using ID generator INSERT INTO teams (name ) VALUES ( 'Liverpool' ) - ID 7 is assigned Insert ID 8 explicitly INSERT INTO teams VALUES ( 8, 'Everton' ) If you insert an ID value explicitly, it has no effect on the sequence generator, and its next value remains unchanged and will be used when you insert subsequent rows: SERIAL - Specify Initial Value and Increment ERROR: null value in column "id" violates not-null constraint INSERT INTO teams VALUES ( 0, 'Reserved' ) INSERT INTO teams VALUES ( NULL, 'Some team' ) In MySQL these 2 values force ID generation, but this is not applied to PostgerSQL: Note that you cannot insert NULL, but can insert 0. Specify DEFAULT INSERT INTO teams VALUES ( DEFAULT, 'Manchester City' ) Omit serial column INSERT INTO teams (name ) VALUES ( 'Aston Villa' ) To generate a ID value, you can omit the SERIAL column in INSERT statement, or specify DEFAULT keyword: If you need a SERIAL column to be unique, you have to specify UNIQUE or PRIMARY KEY explicitly. Id INT NOT NULL DEFAULT NEXTVAL ( 'teams_id_seq' ) , is equivalent to CREATE SEQUENCE teams_id_seq ( lz4 is available only if -with-lz4 was used when building PostgreSQL.When you define a SERIAL column, PostgreSQL automatically changes column to NOT NULL, creates a sequence tablename_serialcol_seq and DEFAULT NEXTVAL to select ID values from the sequence only if they are not supplied in INSERT statement: The supported compression methods are pglz and lz4. (See ALTER TABLE for information on column storage modes.) Setting this property for a partitioned table has no direct effect, because such tables have no storage of their own, but the configured value will be inherited by newly-created partitions. Compression is supported only for variable-width data types, and is used only when the column's storage mode is main or extended. The COMPRESSION clause sets the compression method for the column. Use of EXTERNAL will make substring operations on very large text and bytea values run faster, at the penalty of increased storage space. EXTENDED is the default for most data types that support non- PLAIN storage. Writing DEFAULT sets the storage mode to the default mode for the column's data type. EXTERNAL is for external, uncompressed data, and EXTENDED is for external, compressed data. PLAIN must be used for fixed-length values such as integer and is inline, uncompressed. This controls whether this column is held inline or in a secondary TOAST table, and whether the data should be compressed or not. This form sets the storage mode for the column.
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