Query plan optimization for large payload columns

A computer program product comprising:

• one or more computer readable storage media and program instructions stored on said one or more computer readable storage media, said program instructions comprising instructions to:
• receive a query plan, said query comprising a first phase query plan and a second phase query plan;
• identify a plurality of qualified relevant rows, based on the first phase query plan, each of said plurality of qualified relevant rows comprising one or more payload columns;
• identify one or more encoding candidate payload columns, based on determining those of said plurality of qualified relevant rows that are not encoded;
• analyze said relevant rows of said candidate payload column to yield a count of distinct contents and a payload column width;
• estimate a cost, said cost comprising an estimated size for an encoding dictionary sufficient to perform on-the-fly encoding of said plurality of qualified relevant rows of said payload columns, said estimated size being based on said count of distinct contents, said payload column width, and a hash table bucket size for said encoding dictionary;
• determine whether said cost is larger than an amount of available memory for on-the-fly encoding all of said plurality of encoding candidate payload columns;
• and
• responsive to said cost being less than said amount of available memory, on-the-fly encode said encoding candidate payload column;
• responsive to said cost being greater than said amount of available memory, on-the-fly encode fewer than all of said encoding candidates so as not to exceed said available memory, and leaving alone one or more remaining encoding candidate payload columns unencoded.

A computer system comprising:

• one or more computer processors;
• one or more computer readable storage media;
• computer program instructions;
• and
• said computer program instructions being stored on said computer readable storage media for execution by at least one of said one or more processors, said computer program instructions comprising instructions to:
• receive a query plan, said query comprising a first phase query plan and a second phase query plan;
• identify a plurality of qualified relevant rows, based on the first phase query plan, each of said plurality of qualified relevant rows comprising one or more payload columns;
• identify one or more encoding candidate payload columns, based on determining those of said plurality of qualified relevant rows that are not encoded;
• analyze said relevant rows of said candidate payload column to yield a count of distinct contents and a payload column width;
• estimate a cost, said cost comprising an estimated size for an encoding dictionary sufficient to perform on-the-fly encoding of said plurality of qualified relevant rows of said payload columns, said estimated size being based on said count of distinct contents, said payload column width, and a hash table bucket size for said encoding dictionary;
• determine whether said cost is larger than an amount of available memory for on-the-fly encoding all of said plurality of encoding candidate payload columns;
• and
• responsive to said cost being less than said amount of available memory, on-the-fly encode said encoding candidate payload column;
• responsive to said cost being greater than said amount of available memory, on-the-fly encode fewer than all of said encoding candidates so as not to exceed said available memory, and leaving alone one or more remaining encoding candidate payload columns unencoded.